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REPORT NO. 51 


OF THE 


OPERATIONS EVALUATION GROUP 


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1 his document contains information affecting the national defense of tlie United 
States within the meaning of the Espionage Act, 50 U. S. C., 31 and 32, as 
amended. Its transmission or the revelation of its contents in any manner to an 
unauthorized person is prohibited by law. 

This volume is classified CONFIDEN ITAL in accordance with security regula- 
tions of the War and Navy Departments. 


Manuscript and illustrations for this volume were prepared for pub- 
lication by the Summary Reports Group of the Columbia University 
Division of War Research under contract OEMsr-1131 with the Office 
of Scientific Research and Development. This report was printed and 
bound by the Columbia University Press and was also issued as 
Volume 3 of Division 6 in the series of Summary Technical Reports 
of the National Defense Research Committee. It was originally pub- 
lished in duplicated form on 10 April, 1946, and was given wide 
distribution within the Navy Department. 

Distribution of this volume has been made by the Chief of Naval 
Operations. Inquiries concerning the availability and distribution of 
this report and microfilmed and other reference material should be 
addressed to the Office of the Chief of Naval Operations, Navy 
Department, Washington 25, D. C. 


OEG REPORT NO. 51 


ANTISUBMARINE WARFARE 

IN 

WORLD WAR II 


CHARLES M. STERNHELL AND ALAN M. THORNDIKE 


OPERATIONS EVALUATION GROUP 
OFFICE OF THE CHIEF OF NAVAL OPERATIONS 
NAVY DEPARTMENT 


WASHINGTON, D. C., 1946 


CO WriDEW ^L 





FOREWORD 


ri^ HIS volume embodies the results of some of the 
A statistical and analytical work done during the 
period 1942-45 by members of the Anti-Submarine 
Warfare Operations Research Group of the U. S. Navy, 
later the Operations Research Group and, since Sep- 
tember 1945, the Operations Evaluation Group. The 
group was formed and financed by the Office of Scien- 
tific Research and Development at the request of the 
Navy, and was assigned to the Headquarters of the 
Commander in Chief, U. S. Fleet. The group has been 
of assistance in: 

a. The evaluation of new equipment to meet mili- 
tary requirements. 

b. The evaluation of specific phases of operations 
from studies of action reports. 

c. The evaluation and analysis of tactical problems 
to measure the operational behavior of new material. 

d. The development of new tactical doctrine to meet 
specific requirements. 


e. The technical aspect of strategic planning. 

f. The liaison for the Fleets with the development 
and research laboratories, naval and extra-naval. 

This book consists of a statistical review of the anti- 
submarine war in the Atlantic from 1941 to 1945, to- 
gether with a unified analysis of the tactics which 
proved most useful in this combat. It is believed that 
this material will be of considerable help in providing 
basic understanding of this extremely important branch 
of naval warfare. 

Certain aspects of the work of the group, which 
impinge on the antisubmarine problem, such as the 
Theory of Search and Screening and the more general 
Methods of Operations Research, will be embodied in 
other volumes to be published later. 

Philip M. Morse 

Director, Operations Evaluation Group 


vii 


Cyi'il'lUt-OTI'jL 



t 


PREFACE 


T his volume on antisubmarine warfare [ASW] rep- 
resents a compromise between two major aims, to 
produce a unified summary of the events and prob- 
lems of the antisubmarine war on the one hand, and 
to illustrate the scientific evaluation of naval opera- 
tions on the other. The approach is fundamentally 
historical on both accounts, however, since the illus- 
trations of scientific evaluation are taken from vari- 
ous analyses and studies made in connection with 
antisubmarine warfare during World War II. Great 
care should therefore be exercised in making predic- 
tions concerning the future of ASW from it. There is 
no guarantee that the antisubmarine measures suc- 
cessful in the past will continue to be adequate in the 
future. 

A clear understanding of the events of World War 
II, their reasons and consequences, is necessary, how- 
ever, as background for any decisions which are to be 
made in the postwar period. It is hoped that this vol- 
ume may serve to some extent as a convenient refer- 
ence and source of factual material. One overall con- 
clusion is clearly evident from it: the introduction of 
new weapons, gear, and tactics has led to a continual 
interplay of measures and countermeasures in which 
no other conclusion retains its validity for very long. 
If this lesson alone is learned from it, the volume will 
have served a useful purpose. 

The general organization corresponds closely to 
the dual aim described above, with two parts quite 
different in character. Part I is a historical summary 
of the progress of enemy submarine operations and 
Allied antisubmarine operations during World War 
II. No attempt has been made, however, to give a 
complete chronology of all events. The point of view 
is statistical, and every effort is made to describe the 
progress of World War II in quantitative and objec- 
tive terms. The data are interpreted in terms of the 
ever-changing tactical and strategical situation. Ac- 
cordingly, the historical summary is divided into 
seven chronological periods, as indicated in the 
Table of Contents. This division is necessary because 
of the radical changes in the nature of the U-boat war 
due to changes in U-boat tactics and the introduction 
of new weapons and countermeasures. 

The periods were chosen in such a way that U-boat 
strategy and tactics were fairly homogeneous in each. 
The tactics of individual U-boats varied consider- 


ably, however, from the typical characteristics which 
were common to the operations of most of the 
U-boats during the periods in question. In particular 
the division points between periods are somewhat in- 
definite and represent the approximate dates at which 
a majority of the U-boats had made a major change in 
their methods of operation. 

The data for a given period are further divided 
into three main sections. 

1 . A chronological narrative of the most important 
activities and accomplishments of the U-boats and 
Allied antisubmarine forces. 

2. A more detailed account of the main Allied 
countermeasures to the U-boats, subdivided into 

a. Convoys 

b. Aircraft 

c. Scientific and technical 

d. Sinkings of U-boats. 

3. A survey of the results achieved during the 
period, from both the U-boats’ and the Allies’ point 
of view. This section attempts to explain the reasons 
for the new tactics which characterize the new period. 

Part II of the volume is a more detailed analysis of 
certain of the major problems of ASW, in particular 
those which were the subject of operations research 
studies. The emphasis is on the evaluation of tactics 
and materiel both by theoretical analyses and by spe- 
cial studies of operational data. Although the prin- 
ciples of ASW derived from such evaluation are 
strictly applicable only to the situation which ob- 
tained during World War II, the methods of evalua- 
tion are of more general interest. 

The U-boat war spread over all the oceans of the 
world, but the main battle was fought in the Atlantic. 
Consequently U-boat activity in other regions has not 
been discussed as completely as the Battle of the At- 
lantic. A standard subdivision into areas, which has 
been used throughout the text, is given in the frontis- 
piece. 

There are, in fact, many aspects of the war which 
are omitted from the discussion. The operations of 
midget submarines and small battle craft are gener- 
ally excluded, since they are not considered U-boats. 
The importance of the training of personnel and de- 
velopments along this line are not considered. The 
activities of Naval Intelligence in obtaining informa- 
tion upon which operations are planned are not de- 




ix 


X 


PREFACE 


scribed in any detail. The indirect effects of factors 
such as strategic bombing are largely neglected. The 
net result is to limit the discussion fairly closely to 
Navy antisubmarine operations, though Royal Air 
Force Coastal Command aircraft and those of the 
United States Army Air Forces are included when fly- 
ing antisubmarine missions. The distinction is not a 
hard and fast one. 

The sources of material used are so widely scattered 
through correspondence and informal memoranda 
that it has not been practical to quote references to 
them. Much of the basic information is available in 
periodicals such as the United States Fleet Anti-Sub- 
marine Bulletin, British Admiralty Monthly Anti- 
Submarine Review, and RAF Coastal Command Re- 
\ iew, which have been given wide distribution. Other 
reports, such as Operations Research Group Memo- 
randa and Research Reports and British reports 
originating with Director Naval Operations Research 


[DNOR] and the Operations Research Section of 
Coastal Command [ORS/CC] have been given lim- 
ited distribution and are less accessible, fn addition, 
numerous letters, notes, informal memos, and even 
oral conversations go to make up the background of 
this volume. One of its chief aims has been to set 
down in writing a fair sample of this store of material, 
whose previous status verged on that of folklore. 

No effort has been made, therefore, to assign credit 
for the work discussed. It originates with various 
members of British and United States operations re- 
search groups, military services, and civilian war 
agencies. We have tried to collect available informa- 
tion and tell a reasonably unified story, not of the 
accomplishments of a particular group, but of the 
progress of a special type of naval warfare. 

C. M. Sternhell 
A. M. Thorndike 
' Editors 



C 


CONTENTS 


PART I 

HISTORY OF ANTISUBMARINE OPERATIONS 

CHAPTER page 

1 First Period— Submerged Daylight Attacks on Independ- 
ents, September 193 9 -June 1940 3 

2 Second Period— Night Surfaced Attacks on Convoys, July 

1940-March 1941 8 

3 Third Period-Start of Wolf Packs; End-to-End Escort of 

Convoys, April 1941-December 1941 16 

4 Fourth Period— Heavy Sinkings on East Coast of United 

States, January 1942-September 1942 25 

5 Fifth Period — Large Wolf Packs Battle North Atlantic 

Convoys, October 1942 -June 1943 34 

6 Sixth Period— Aircraft Defeat U-Boats’ Attempted Come- 
Back and Force Adoption of Maximum Submergence, 

July 1943 -May 1944 44 

7 Seventh Period — Schnorchel U-Boats Operate in British 

Home Waters, June 1944-End of War 64 

8 Summary of Antisubmarine Warfare, World War II . . 80 

PART II 

ANTISUBMARINE MEASURES AND THEIR 
EFFECTIVENESS 

9 Safety of Independent Shipping 93 

10 Convoying and Escort of Shipping 100 

1 1 Attacks by Surface Craft 113 

12 Attacks by Aircraft 127 

13 Offensive Search I39 

14 Employment of Search Radar in Relation to Enemy 

Countermeasures I53 

15 Countermeasures to the German Acoustic Torpedo . 161 

Epilogue 177 

Appendix I 181 

Glossary 183 

Index 191 





Ocean Areas Referred to in the Text 







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AZA 

Azores Area 

CSW 

Caribbean Sea Frontier — West 

NSA 

North Sea Area 

BCA 

Biscay Channel Area 

ESF 

Eastern Sea Frontier 

NTE 

Northern Transit Area — East 

BDA 

Bermuda Area 

FRA 

Freetown Area 

N FW 

Nojrthern Transit Area — WTst 

BLA 

Baltic Area 

GMA 

(iibraltar Morocco Area 

NWA 

Northwest Atlantic Area 

BTA 

Barents Sea Area 

GSF 

Gulf Sea Frontier 

PSF 

Panama Sea Frontier 

BZA 

Brazilian Area 

IND 

Indian Ocean 

SEA 

Southeast Atlantic Area 

CCZ 

CSE 

Canadian Coastal Zone 

Caribbean Sea Frontier — East 

MED 

NEA 

Mediterranean Red Sea Area 
Northeast AUantic Area 

SWA 

Southwest Atlantic Area 


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L 



PART I 

HISTORY OF ANTISUBMARINE OPERATIONS 


SUBMARINES IN WORLD WAR I 

T he great capabilities of the submarine as a 
weapon of war were first revealed during World 
War I when the U-boat campaign almost proved de- 
cisive. Fortunately, the Germans themselves did not 
fully realize in 1914 how valuable the U-boat's ability 
to submerge and escape detection would be for offen- 
sive operations against enemy shipping. The small 
number of U-boats available to the Germans were 
used at first only to attack naval ships and it was 
not until 1915 that a concerted attack was begun on 
English merchant shipping. 

During 1915 and 1916 there were on the average 
only about 15 U-boats at sea at any time. These U- 
boats were sinking about 200,000 gross tons of ship- 
ping a month, while about U-boats were being 
sunk each month. This situation was extremely satis- 
factory to the Germans, as the average life of a U- 
boat at sea during this period was about 10 months, 
during which the U-boat would sink about 13,000 
gross tons of shipping a month, for a total of 130,000 
gross tons of shipping sunk before the U-boat itself 
was sunk. 

Encouraged by these successes, the Germans in 
February 1917 started a large scale campaign of un- 
restricted warfare on merchant shipping in an at- 
tempt to blockade England. This attempt almost 
proved successful as Allied shipping losses rose 
steadily to a peak in April 1917. Four hundred and 
forty-four ships of about 900,000 gross tons were sunk 
by U-boats during that month. The British Fleet was 
confined to its bases for there was only 8 weeks’ sup- 
ply of fuel oil in England. Various countermeasures 
had been tried without success and defeat seemed 
just around the corner unless an antidote to the 
U-boat could be found. 

INTRODUCTION OF CONVOYING 

Admiral Jellicoe was brought to Admiralty to deal 
with the situation. The convoy system, twice turned 
down on account of lack of escort vessels and loss of 
time to shipping, was introduced in April 1917 and 
proved immediately successful in reducing the ship- 
ping loss rate. The result of all the various British 


countermeasures, of which the convoy system was the 
most effective, was that by October 1917, 1501 ships 
in 99 convoys had been brought into port with the 
loss of only ten ships sunk while in convoy (a loss 
rate of less than 1 per cent). 

LACK OF SATISFACTORY 
COUNTERMEASURES 

Although shipping losses had been checked, it 
should be kept in mind that, from an offensive point 
of view (i.e., destruction of U-boats), the U-boat had 
not been definitely beaten in World War I. After the 
start of unrestricted U-boat warfare early in 1917, the 
Germans maintained an average of about 40 U-boats 
at sea at any time. During this period the average 
number of U-boats being sunk each month was only 
about seven; the maximum number of U-boats sunk 
in any month was only 14 in May 1918. Therefore, 
the average life of a U-boat at sea during the last year 
of World War I was still about 6 months. Shipping 
losses, even during the last year of World War I, were 
still running at the level of about 300,000 gross tons 
a month, so that at that time each U-boat was still 
sinking about 45,000 gross tons before it, itself, was 
sunk. 

These figures indicate that other factors besides 
U-boat losses must have contributed to the mutiny 
of U-boat crews in 1918, as the rate of U-boat losses 
has reached far higher levels in World War II with- 
out any corresponding crack-up in morale. Another 
point to be considered is that a large part of German 
U-boat losses in the latter part of World War I was 
due to mines, whose effectiveness was greatly in- 
creased by the fact that the geographical position of 
the German U-boat bases necessitated passage 
through the North Sea. Of the 178 U-boats sunk dur- 
ing the first World War, about 45 per cent were sunk 
by surface craft, about half of these by depth charges 
and half by gunfire and ramming. About 30 per cent 
were sunk by mines, another 10 per cent were tor- 
pedoed by submarines, and the other 15 per cent by 
other causes. It is therefore clear that the Allies had 
not developed any offensive weapon during World 
War I which could deal so effectively with the U-boat 
at sea that further operations would not be profitable. 


2 


HISTORY OF ANTISUBMARINE OPERATIONS 


That the Germans themselves still thought the 
U-boat was an effective weapon at the end of World 
A\^ar I may be seen from the fact that there were 
about 220 U-boats under construction in November 
1918. Admiral Scheer’s building program of October 
1, 1918 provided for at least 30 U-boats a month be- 
ginning the middle of 1919 and would probably have 
been fulfilled if hostilities had continued. If the war 
had not ended in November 1918, the Allies would 
have had to face a second and more intensive U-boat 
campaign. 

NEED FOR SCIENTIFIC AND 
TECHNICAL ADVICE 

One of the most significant points about antisub- 
marine warfare which became apparent early in 
World War 1 was the necessity of having scientific 
and technical aid in combatting the U-boat. The es- 
sential problem was that of having some means of 
detecting a submerged U-boat and then of having 
some weapon that would provide a good chance of 
destroying the U-boat. 

The first crude attempt to develop an instrument 
to detect the submerged U-boat resulted in the in- 
stallation of hydrophones on Allied naval ships in 
1915. The hydrophone was simply an instrument for 
listening to the noise produced by the submarine, 
and sonic frequencies below 10 kc were used. No 
range and only a rough bearing were obtained from 
these early hydrophones and it was impossible to 
make attacks on U-boats with any degree of precision. 
The main effect of hydrophones was on U-boat 
morale, as U-boats found that they were being fol- 
lowed after diving instead of being free of their 
pursuers. 

The first depth charges to be used in attacking 
submerged U-boats were also introduced in 1915. 
However, so few were available that the Germans 
did not realize they were being used until 1917. 

In September 1918 the British formed a small com- 
mittee, consisting largely of scientists, called the 
Anti-Submarine Division International Committee 
(the initials spell ASDIC, the name given by the 
British to their echo-ranging detector). This com- 
mittee developed a method of transmitting sound of 
supersonic frequencies under water and then using 
the echo returning from the submerged submarine 
to fix its position. Although the Asdic set was still in 
the experimental stage when World War I ended. 


the labors of the committee were not wasted, as effec- 
tive underwater echo-ranging gear was developed in 
the 1930’s and proved to be quite a surprise to the 
Germans at the start of World War II. Due to the 
ability of Asdic to provide both range and bearing, it 
proved far better than the hydrophones used in 
World War I. Hydrophones, themselves, were also 
improved by using supersonic frequencies and mak- 
ing them directional, thereby enabling the operator 
to obtain more accurate bearing. 

ORDER OF BATTLE -SEPTEMBER 1939 

At the start of World War II, England had only 
about 220 Asdic-fitted antisubmarine craft consisting 
of approximately 165 destroyers, 35 patrol craft {i.e., 
sloops, frigates, corvettes) and 20 trawlers. This total 
may be compared with the more than 3000 ships 
(about 450 destroyers, 170 patrol craft and the re- 
mainder trawlers and small craft) available to the 
Allies for antisubmarine warfare in 1918. 

The British, profiting from their experience in 
World War I, had learned that the ocean convoy 
system did more than anything else to reduce ship- 
ping losses. They knew that the convoy system works 
best in open water where evasion can be employed 
and that its success depends upon efficient escorts 
armed with effective offensive weapons. They were 
also aware of the fact that an efficient U-boat tracking 
system is necessary to practice effective evasion, and 
a daily U-boat plot based on contacts, DF fixes, and 
intelligence was used throughout the war. 

Meanwhile the Germans had done considerable re- 
search in developing and improving their U-boats. 
The U-boats available to the Germans at the start of 
World War II were faster than those used in World 
War I and were also considerably stronger, being 
able to dive deeper and to withstand more depth- 
charge punishment. The Germans had also devel- 
oped an electric torpedo which left no visible wake. 

However, in September 1939, the Germans seem to 
have had available only about 60 U-boats, of which 
30 were of the small 250-ton type (of limited endur- 
ance-suitable for coastal operations only) and 30 of 
the larger ocean-going type, 20 of which were of 500 
tons and 10 of 750 tons. This small number suggests 
that Germany, possibly not anticipating that Eng- 
land would enter the war at that early date, had given 
higher priorities to the building of tanks and aircraft 
for land warfare than to the building of U-boats. 


Chapter 1 

FIRST PERIOD 

SUBMERGED DAYLIGHT ATTACKS ON INDEPENDENTS 
SEPTEMBER 1939 -JUNE 1940 


U-BOAT OFFENSIVE 

T his first phase of U-boat^ warfare was greatly in- 
fluenced by the rapidly changing overall military 
situation. Germany invaded Poland on September 1, 
1939, and England and France declared war on Ger- 
many on September 3. Some U-boats had left Ger- 
many early in August and when the war began there 
were about six at sea, ready to start an offensive in 
the Northeast Atlantic in the Western Approaches 
to England. 

According to statements of early prisoners of war, 
the commanding officers of U-boats had been ordered 
to observe International Law, which forbade U-boats 
to sink merchant vessels without having first 
placed the passengers and crew in a place of safety. 
At the beginning of September, these instructions 
seem to have been generally obeyed, with the notable 
exception of the Athenia, which was torpedoed with- 
out warning on September 3. However, this situation 
did not last long and, toward the end of September, 
even neutral ships were being torpedoed without 
warning. 

Anticipating unrestricted U-boat warfare, the Brit- 
ish had prepared plans before the war for the imme- 
diate establishment of the convoy system and the first 
trade convoy sailed on September 6. As the British 
defenses against U-boat attacks were based on the 
needs of protecting primarily the fleet and second- 
arily merchant shipping, the limited number of anti- 
submarine vessels available for convoy escort was 
inadequate to provide direct protection to the con- 
voys. Nevertheless, it was believed that the British 
antisubmarine measures were sufficiently effective to 
ensure that no U-boat could betray her presence by 
attacking a convoy without running a severe danger 
of subsequent destruction by the escorting craft. 

The experience during September tended to jus- 
tify these expectations, as over 900 ships were con- 

a The term U-boat is used to refer to any enemy submarine 
(German, Italian, Vichy French, or Japanese) with a displace- 
ment of 200 tons or more. 


voyed during the month without the loss of a single 
ship while in convoy. In addition, two U-boats were 
sunk during the month by British surface craft. The 
Germans apparently had no knowledge of British 
Asdic and still believed that they could counter 
underwater detection by reducing internal noises. 

The lack of knowledge of British Asdic probably 
accounted for the early U-boat tactics. The U-boats 
preferred attacking their targets during the daylight, 
believing themselves relatively invisible because of 
their powers of submergence, while they could ob- 
serve the targets through their periscopes. The U-boat 
attacks were generally made by torpedo from peri- 
scope depth, but if the target was an unarmed mer- 
chant vessel, the U-boat usually surfaced and at- 
tempted to sink the ship by gunfire. 

During September, while the convoy system was 
still not fully established, there was a sufficient num- 
ber of unescorted targets at sea to enable the U-boats 
to sink 39 ships of 151,000 gross tons. Ten of these 
ships were sunk by gunfire alone, from surfaced 
U-boats, and this led the British to take immediate 
steps to arm as many merchant ships as possible to 
defend themselves against such attacks. 

At the start of the war antisubmarine forces in the 
Western Approaches were augmented by aircraft car- 
riers, but after HMS Courageous was sunk by U-boat 
torpedoes on September 17, the carriers were with- 
drawn. However, shore-based aircraft of the Goastal 
Command helped considerably by flying over 100,000 
miles in September, sighting some 50 U-boats or sup- 
posed U-boats, and attacking over 30 of them. Al- 
though none of the aircraft attacks were very effec- 
tive, they did cause the U-boats to submerge and 
thereby reduced their effective operating period. 

The September U-boat campaign was followed by 
a lull during the first ten days of October during 
which, although U-boats were at sea, hardly any ships 
were attacked. This lull seemed to reflect the political 
situation at the time, as it was accompanied by Hit- 
ler’s offer of peace on October 6. U-boat activity 
flared up again on October 12, and by the end of the 




4 


SUBMERGED DAYLIGHT ATTACKS ON INDEPENDENTS 


month 28 ships of 136,000 gross tons had been sunk 
by U-boats. In addition Kapitan-leutnant Prien, in 
command of U-47, penetrated the harbor of Scapa 
Flow in the middle of October, and sank HMS Royal 
Oak, a British battleship. This served to direct Brit- 
ish attention to the necessity of protecting harbors 
against U-boats by means of fixed defenses, such as 
booms, indicator loops, mine fields, and harbor de- 
fense Asdics. 

During November and December the main effort 
of the German U-boats seems to have centered upon 
a mine-laying campaign on the East Coast of Eng- 
land, particularly in the Thames Estuary. The mines 
laid were both the old type of contact mine and a 
new type of magnetic mine, which at first proved 
rather difficult to sweep. Monthly losses due directly 
to U-boats (torpedoes and gunfire) fell to 18 ships of 
about 65,000 gross tons and were exceeded by the 
100,000 gross tons of shipping sunk by mines during 
each of these months. 

U-boat activity began increasing again in the sec- 
ond week of January 1940 and by the end of the 
month there were as many U-boats at sea as at the 
start of the war. In February, the U-boat effort was 
greater than during any previous period and 35 ships 
of 153,000 gross tons were sunk. The U-boats con- 
tinued to follow a policy of attacking Allied %nd 
neutral ships without warning. They preferred at- 
tacking single ships or stragglers from convoys, thus 
making it difficult for the antisubmarine ships to con- 
duct an effective search and counterattack. The re- 
spect the U-boats had been showing for the Briti^ 
convoys is indicated by the fact that only 7 of the 169 
ships sunk by U-boats during the first six months of 
the war were in convoy when sunk, although roughly 
half the shipping sailed in convoy at this time. 

Losses due to mines fell off during January and 
February as better methods of sweeping the magnetic 
mines were developed and more ships were degaussed 
(magnetic field of ship changed to protect it against 
magnetic mines). 

There was a marked lull in U-boat activity 
throughout March, featured by the complete absence 
of U-boats from Atlantic waters after about the 12th 
of the month. Early in April, every available U-boat 
left Germany to take up patrol positions in the North 
Sea to help in the impending military operations 
against Norway. The average number of U-boats at 
sea reached a peak of about 15 during the second 
week of April, when Germany invaded Norway. De- 


spite the large concentration of U-boats, the damage 
done by them was remarkably small. No British 
capital ship was even attacked by U-boats and only 
six ships of 31,000 gross tons were sunk by the U-boats 
during the whole month of April, a new low for the 
war. In addition, the Germans lost six U-boats during 
the month, a new high for the war. 

There was very little U-boat activity during the 
first half of May as Germany started her invasion of 
Holland and Belgium on May 10. It is believed that 
during May no U-boat proceeded to the Western 
Approaches until the 21st and only 10 ships of 48,000 
gross tons were sunk by U-boats during the month. 
Shipping losses to U-boats were exceeded for the first 
time during the war by the 154,000 gross tons sunk 
by aircraft. These losses were incurred largely in con- 
nection with the operation and evacuation of the 
British Expeditionary Force, which left Dunkerque 
on May 29. 

The Germans announced on May 29 that U-boat 
warfare was about to recommence and warned neu- 
trals not to enter the protection of British convoys. 
This threat was followed by a period of intense 
U-boat activity as convoys were attacked with greater 
boldness than in earlier periods, advantage being 
taken of the paucity of escorts, rendered inevitable 
by the demands of the military evacuation and the 
Home Fleet. The losses for June were the highest of 
the war, with 56 ships of 267,000 gross tons being sunk 
by U-boats. The German ace. Kit. Prien, contributed 
his share by sinking ten ships of about 67,000 gross 
tons during one cruise. By the end of June, France 
was out of the war and Italy had entered the war 
with over 100 U-boats, about 60 of which were 
ocean-going (650 tons and over). 

12 COUNTERMEASURES TO THE 

U-BOAT 

Convoys 

The 'convoy system was by far the most effective 
countermeasure in keeping down shipping losses to 
U-boats during this first period, just as it had been 
during World War I. This was still true, even though 
the number of antisubmarine vessels suitable for 
ocean escort was insufficient to provide direct protec- 
tion to the convoys. The British met this problem by 
keeping their convoy system flexible, changing the 
number of escorts and the distances for which con- 


COUNTERMEASURES TO THE U-BOAT 


5 


voys were escorted in accordance with U-boat activ- 
ity. It should be noted that the Germans made this 
problem more difficult by sending the U-boats out in 
waves, so that peaks of U-boat activity occurred in 
September 1939 and in February and June of 1940. 

Although the first convoys sailed early in Septem- 
ber 1939, the convoy system was not fully in force 
until the beginning of October. The designations of 
the main convoy routes that were set up then were: 

OB Outward bound from England to America 
and Africa. 

HX Homeward-bound to England from Halifax. 

SL Homeward bound to England from Sierra 
Leone. 

In order to illustrate some of the problems in- 
volved in setting up the convoy system a detailed 
account is presented of the changes made in the HX 
convoy route during this period. On October 7, 1939, 
it was decided to discontinue the convoys from Kings- 
ton, Jamaica, and all ships in the West Atlantic were 
routed independently to Halifax, taking advantage 
of U. S. waters as far as possible. Convoys were di- 
vided into slow (9- to 12-knot) and fast (12- to 15-knot) 
convoys, which left Halifax at about the same time in 
order to arrive four days apart at the rendezvous 
point. At this point, located at about 15° west longi- 
tude, the convoys were met by one or two destroyers 
which provided antisubmarine escort to England. 
The ocean escort, provided between Halifax and the 
rendezvous point primarily for protection against 
surface raiders, consisted of a battleship, cruiser, or 
armed merchant cruiser, and one or two submarines 
when available. 

The first of these convoys, HX 6 and HXF 6, con- 
sisted of 62 and 6 ships, respectively. The dividing 
line was then altered to 1 1 knots to equalize the 
number of ships, and during November 1939 the 
number of ships in these sections averaged 32 and 12. 
On February 12, 1940, the fast convoys were discon- 
tinued, and all HX convoys sailed at 9 knots, at 3- and 
5-day intervals. These convoys consisted of ships with 
speeds between 9 and 15 knots, ships of higher speeds 
sailing independently. At the beginning of April, in 
order to equalize the size of the convoys, 4-day inter- 
vals were started. 

Early in May 1940 Bermuda began to be used as an 
assembly point for vessels from the West Indies and 
other points in that vicinity, and HX 41 was the first 
combined Bermuda and Halifax convoy. The sec- 


tions formed at sea, as arranged, at about 41° north 
latitude and 43° west longitude, and the Bermuda 
escort then returned to base. This change enabled 
about 60 per cent of the ships that formerly sailed 
from Halifax to cut down their voyage by 500 miles 
and to avoid the fog off Newfoundland. The average 
number of ships in these HX convoys had risen to 46 
by May 1940. 

In addition to the above-mentioned convoys, the 
British also sailed coastal convoys to protect shipping 
on short trips around the English coast and Scandi- 
navian convoys to and from Norway. The main 
energies of the French light craft were also devoted 
to the protection of merchant shipping. They were 
fitted with Asdic as soon as possible after the opening 
of the war and provided escorts for purely French 
convoys, helped escort the Gibraltar convoys for 
most of the way, and assisted in covering the military 
cross-Channel convoys. 

The extreme value of the British convoy system 
may best be appreciated by noting that during this 
period about 2500 ships were being convoyed 
monthly, while only about 5 of these were being sunk 
monthly by U-boats (2i/^ in escorted convoys, II /2 in 
unescorted convoys, and 1 straggler). The rate at 
which independent merchant vessels were being sunk 
b)4U-boats was roughly about four times as high. 

Aircraft 

Another important countermeasure to the U-boat 
^as the use of aircraft. These had seen very little use 
against U-boats during World War I and conse- 
quently it took some time before the problems of how 
to use aircraft most efficiently against U-boats were 
solved. In addition, the aircraft were still armed only 
with bombs. Consequently the direct contribution of 
aircraft toward sinking U-boats was negligible dur- 
ing this period. 

Nevertheless, aircraft performed a defensive func- 
tion of great value in helping to protect shipping. 
Coastal Command aircraft flew, on the average, 
about 4500 hours monthly on purely antisubmarine 
work. About 20 U-boats were sighted monthly and 
12 of these were attacked, with about 10 per cent of 
the attacks resulting in some damage to the U-boat. 
This effort reached a peak of 9500 hours during 
June 1940, when about 2800 hours were spent on 
antisubmarine patrol and 6700 hours on convoy 
escort duty. 


6 


SUBMERGED DAYLIGHT ATTACKS ON INDEPENDENTS 


The main value of this flying was in causing the 
U-boats to submerge, thus preventing them from 
shadowing or approaching convoys on the surface. 
It also helped to discourage them from operating 
close to the shores of England where the flying was 
heaviest. U-boats at this time were under orders to 
submerge as soon as they sighted a })lane and the 
British took advantage of this by starting to use, in 
November 1939, light aircraft of the Moth type to 
patrol around the coast. These aircraft were known 
as “scarecrows,” carried no bombs, and were used 
solely to sight and report U-boats, and to make them 
submerge. These flying hours and sightings also 
helped considerably in keeping an accurate U-boat 
plot. 

Scientific and Technical 

Applying the lessons learned in World War I, con- 
siderable scientific work was being done during this 
period to improve antisubmarine attacks. Some of 
the typical problems being investigated then were: 

1. Development of an Asdic receiver-amplifier 
with automatic sensitivity control so that both long 
and short range echoes would be clearly recorded. 

2. Theoretical investigation of improved methods 
of carrying out antisubmarine attacks and of the best 
type of depth-charge pattern to ensure destruction of 
the submarine, 

3. Assistance to antisubmarine personnel in dis- 
tinguishing between submarine and non-submarine 
targets, as a great amount of effort and a large num- 
ber of depth charges were being expended on wrecks, 
whales, and other non-submarine targets. 

Sinking of U-Boats 

Surface craft, equipped with Asdic and depth 
charges, were by far the most potent enemy of the 
U-boat during this first phase of U-boat warfare. 
Twenty-one German U-boats are known to have 
been sunk^ as a result of Allied action during this 
10-month period; 15 were sunk by surface craft, one 
by the coordinated action of two ships and one plane, 
one by a plane from a British battleship, two were 
torpedoed by submarines, and two were mined in 

b The estimates given here for U-boat sinkings are based on 
Allied assessments. Incidents assessed A or B are considered to 
have sunk the U-boat. Justification for this assumption is given 
in Appendix 1. 


attempting to pass through the Dover Barrage in 
October. Two other German U-boats were sunk 
under unknown circumstances while one is known to 
have been sunk in the Baltic after being rammed 
accidentally. 

In addition to the 24 German U-boats mentioned 
above, 10 Italian U-boats were sunk in the Mediter- 
ranean, Red Sea, and Indian Ocean between June 
10, when Italy entered the war, and the end of the 
month. 

13 SURVEY OF RESULTS 

1.3.1 From the U-boat Point of View 

The average number of U-boats at sea in the Atlan- 
tic during this first phase of the U-boat war was about 
six. The average number of ships sunk monthly by 
them was 26 of about 106,000 gi oss tons, so that about 
four ships of about 18,000 gross tons were being sunk 
per U-boat month at sea. However, about two out of 
the six U-boats at sea were being sunk each month, 
so that the average life of a U-boat at sea was only 
about three months. This relative rate of loss of 
U-boats was extremely high, much higher than at 
any stage of the first World War, and makes readily 
understandable the fact that they preferred attacking 
unescorted ships to attacking convoys, lightly es- 
corted as they were. It also helps to explain why the 
German U-boats felt it necessary to change their 
tactics during the next phase of the U-boat war; this, 
despite the fact that the overall exchange rate {i.e., 
13 ships of about 53,000 gross tons sunk for each 
U-boat sunk) might be considered satisfactory for the 
U-boats. The rate of loss of U-boats simply was higher 
than the Germans could afford. 

The fact can be clearly seen from another ap- 
proach. The Germans started the war with about 30 
ocean-going U-boats {i.e., 500 tons or larger). By the 
end of June 1940, 18 of these had been sunk while 
only about 15 new ones had been commissioned, so 
that the Germans only had about 27 ocean-going 
U-boats available at the start of the second period of 
the U-boat war. 

1.3.2 From the Allies’ Point of View 

At the end of June 1940 England was left alone in 
the war against Germany and her ability to carry on 
the war was dependent on her keeping her sea lanes 
open. Total shipping losses of the Allied and neutral 


SURVEY OF RESULTS 


7 


nations were about 280,000 gross tons monthly as 
compared to a building rate of only about 88,000 
gross tons monthly, for a total net loss of 1,920,000 
gross tons due to all causes during this 10-month 
period out of a total of about 40,000,000 gross tons of 
shipping at the start of the war. It appeared that 
shipping losses were still on the upgrade and the 
only hope of keeping the rate of net loss down was a 
large increase in shipbuilding. 

Of the 280,000 gross tons of shipping lost monthly, 
about 223,000 gross tons were lost by enemy action, 
with U-boats accounting for 106,000 gross tons or 48 
per cent of the total lost by enemy action. Mines 
accounted for 58,000 or 26 per cent, aircraft for 
27,000 or 13 per cent, surface craft for 14,000 or 6 
per cent, and other and unknown causes for the other 
7 per cent of the losses. 

The U-boat appeared definitely to be the main 
threat to Allied shipping. The convoy system had 


been the main factor in keeping the shipping losses 
due to U-boats down to a moderate level. Although 
the number of British Asdic-fitted antisubmarine 
vessels increased from about 220 at the beginning of 
the war to about 450 at the end of June 1940, most of 
the increase took place in trawlers and other small 
ships. The 450 ships consisted of about 180 de- 
stroyers, about 55 patrol craft, and about 215 trawlers 
and other small craft. However, the number of these 
ships that could be spared for escort duty was still 
insufficient to provide adequate protection to the 
convoys. The British had been fortunate during the 
first period that the enemy had only a small number 
of U-boats available and these had operated in a 
limited area, almost all the sinkings of ships occur- 
ring in the Northeast Atlantic (east of 20° west longi- 
tude and north of 30° north latitude). This had 
helped to make the escort problem easier during the 
first period. 






Chapter 2 

SECOND PERIOD 

NIGHT SURFACED ATTACKS ON CONVOYS 
JULY 1940-MARCH 1941 


2 1 U-BOAT OFFENSIVE 

T he second phase of the U-boat war was marked 
by a complete change in enemy tactics. The Ger- 
mans, having discovered as a result of their high rate 
of loss that the U-boats were quite vulnerable to 
Asdic when submerged, decided to make use of the 
hours of darkness to regain their relative invisibility. 
At night, trimmed down on the surface, a U-boat 
offers a very small target to the human eye and is also 
rather difficult to detect by Asdic. The surfaced 
U-boat has the advantage of high speed and maneu- 
verability and therefore has good chances of avoid- 
ing detection by the escorts. Acting on this principle 
and encouraged by the results achieved at night dur- 
ing the first period by a few of the more successful 
U-boat captains, the U-boats started, in July 1940, 
the general practice of attacking convoys at night 
from a surfaced position and then using their high 
surface speed to escape. Occasional daylight attacks 
were still made on ships sailing independently, and 
on stragglers from convoys. 

Accompanying this change in the enemy’s tactics 
came the occupation of the French ports and their 
establishment as U-boat bases, marked by the first 
visit of a U-boat to Lorient on July 22. The use of 
French bases served to cut down the transit time of 
the U-boats and enabled them to extend their area 
of operations further westward in the Atlantic. From 
his air bases in France, the enemy was also able to 
send out long-range reconnaissance aircraft to pick 
up convoys in the Atlantic. 

In addition, after the fall of France there devel- 
oped the threat of a sea-borne invasion of England. 
This confined a large number of destroyers to the 
East and South coasts of England and consequently, 
as the number of ships available for convoy escort 
was necessarily limited, the U-boats were encouraged 
to attack convoys more frequently. Aircraft were also 
diverted from antisubmarine patrols over the West- 
ern Approaches to England, where the U-boats were 
operating, to anti-invasion patrols to the eastward. 


Some Italian U-boats had also started operation in 
the Atlantic in August 1940. These Italian U-boats 
used Bordeaux as a base and followed the same 
methods as German U-boats, presumably working 
directly under German orders. Their operational 
areas were usually southward of the ones used by the 
German U-boats. 

Increased U-boat activity, which had commenced 
in June 1940, continued throughout July and August 
with over 200,000 gross tons of shipping being sunk 
in each of those months. Up to the middle of July, 
the most active area was still the Western Approaches 
between the latitudes of 48° north and 51° north. 
After the threat of air attack from French bases had 
led to the rerouting of British convoys around the 
north of England, the U-boats lost no time in shifting 
their area of activity to the Northwestern Approaches 
to meet the increased traffic there. This activity was 
marked by increased attacks on convoys while anti- 
submarine escorts were actually present, but these 
attacks were generally on large convoys which, owing 
to the shortage of escort vessels, were guarded by only 
about two Asdic-fitted ships. 

On August 15 Germany proclaimed a complete 
blockade of the British Isles and called upon neutral 
governments to forbid their ships to sail through the 
Anglo-German war zone. U-boat activity was con- 
siderably intensified after that date and the shipping 
losses continued to increase, with about 300,000 gross 
tons being sunk by U-boats in September, and a new 
high for the war was reached in October when 62 
ships of 346,000 gross tons were sunk by U-boats. The 
scene of greatest activity during these months was 
still the Northwestern Approaches, with night at- 
tacks on convoys being the most favored method of 
attack by the U-boat. Of the 59 ships attacked in this 
area in September, 40 were in convoy; 71 per cent of 
the total were night attacks. The concentration of 
aggressive operations into the period of, and imme- 
diately following, the full moon was especially no- 
table during October when 31 ships were attacked on 
October 18 and 19. 


8 


U-BOAT OFFENSIVE 


9 


It should be kept in mind that during these months 
of heavy losses the average number of U-boats at sea 
was still only about six. This means that ten ships of 
about 60,000 gross tons were sunk by the average 
U-boat at sea during October 1940, probably an all- 
time high in operating efficiency for submarines. In 
addition to inflicting these heavy losses, the U-boats 
were almost invariably escaping unscathed, as, for 
example, in October when only one U-boat was sunk 
in the Atlantic. These were the days when the star 
German commanders (U-boat aces) such as Prien 
and Kretschmer were operating. These aces had sur- 
vived the hazards of operating during the first period 
and had profited from the experience gained then. 
The U-boats making these night attacks on convoys 
were operating individually and usually only one or 
two U-boats would be involved in the attack. Despite 
this, some of the convoys suffered rather heavy losses, 
as, for example, HX 79, which lost 12 ships to two 
U-boats in one night in October. 

The normal procedure for U-boats attacking con- 
voys at this time seems to have been as follows: The 
U-boat gained contact with the convoy during the 
day, either as a result of reports from long-range 
German reconnaissance aircraft, reports from other 
U-boats, or by sighting smoke, and then proceeded 
to shadow the convoy at visibility distance on the 
bow or beam. When darkness had fallen, the U-boat, 
trimmed down on the surface, closed the convoy, and 
endeavored to reach a position broad on its bow. She 
kept very careful watch for the escorts and endeav- 
ored to pass astern of those stationed on the bow of 
the convoy. The approach was pressed home as close 
as the U-boat captain dared, and it is possible that, 
in some cases, a firing range of about 600 yards was 
achieved. Having reached a firing position on the 
beam of the convoy, most U-boats increased to full 
speed, fired a salvo of four torpedoes, turned away 
still at full speed, firing stern tubes if possible, and 
retired as rapidly as possible on the surface in the di- 
rection considered safest. If their retreat was unseen, 
they might reload their torpedo tubes on the surface 
and attack again in the same manner later in the 
night. 

The serious damage inflicted on British convoys by 
these new U-boat tactics caused a considerable num- 
ber of changes to be made in the convoys. The spac- 
ing of the convoy columns was opened up to reduce 
the chance of more than one ship’s being hit by a 
salvo. Escorts were stationed farther away from the 


convoy and new plans were developed for searching 
for the U-boat with illuminants following attack. To 
improve the tactical efficiency of the escorts, these 
ships were formed into groups and as far as possible 
ships of one group were to work together. Admiralty 
took over the responsibility for the routing of all 
ocean-going convoys, thus enabling emergency 
changes to be made without delay. In addition, great 
efforts were made to equip all convoy escorts with 
radar, which would enable them to locate U-boats 
on the surface at night outside visibility distance and 
possibly before they could attack the convoy. 

By November 1940 Lorient had become the prin- 
cipal U-boat base and during this month one Ger- 
man U-boat had left this port and gone as far south 
as Freetown, sinking four ships in three days. Novem- 
ber was also marked by several heavy air attacks on 
the ports of Lorient and Bordeaux, which were con- 
sidered to have inflicted severe damage on both 
U-boats and their bases. 

The first known successful counterattack against 
the German method of night attack on convoys oc- 
curred on November 21 after two ships of convoy 
OB 244 had been torpedoed. The British corvette 
HMS Rhododendron, stationed astern as a rescue 
ship, sighted an object momentarily at a range of 
1500 yards about an hour after the torpedoing. 
Three minutes later Asdic contact was gained and 
then depth charges were dropped with the result that 
considerable metallic wreckage and oil were blown 
to the surface. 

This successful counterattack, plus the loss of two 
other U-boats, might account in part for the reduced 
number of attacks on escorted convoys in November 
and December 1940. Heavy winter weather in the 
North Atlantic was probably also a factor in account- 
ing for the decrease in shipping losses to U-boats, as 
only 150,000 gross tons were sunk in November and 
200,000 gross tons in December. 

Early in December a westerly movement of the 
U-boats became noticeable, with most of them sta- 
tioned as far out as 20*^ west longitude. This may have 
been due in part to Coastal Command flying and in 
part to an attempt to intercept incoming convoys 
before the antisubmarine escort joined. However, 
this actually increased the enemy’s difficulty in locat- 
ing convoy traffic. 

As a counter to the fact that U-boats in the North- 
western Approaches had been attacking British con- 
voys in longitudes 20° to 25° west, which is an area 




10 


NIGHT SURFACED ATTACKS ON CONVOYS 


beyond the point which could be reached by the 
escorts, new evasive routing measures were adopted 
in December. It was decided to make use of disper- 
sion to the maximum extent that the endurance of 
merchant ships permitted, and the routes of the con- 
voys were spread between 63i/2® and 57° north lati- 
tude. The cycles of convoys were also opened out, 
with the object of reducing the strain on escorting 
forces. 

This thorough diversion of convoy routes seems to 
have been the main factor in the reduction of ship- 
ping losses, just as it had been in World War I. No 
attacks were made on escorted convoys from Decem- 
ber 1940 until January 29, 1941, and the shipping 
losses to U-boats in January dropped to 21 ships of 
127,000 gross tons, the lowest figure since the Ger- 
mans announced their intensified U-boat campaign 
in May 1940. This occurred despite the fact that the 
average number of U-boats at sea in the Atlantic had 
increased to about 12. Most of the ships lost during 
these two months were not in convoy, since the 
U-boats had difficulty finding convoys and resorted 
to the much easier task of picking off stragglers or 
ships sailing independently. 

The month of February 1941 opened with a con- 
tinuation of the comparative lull in U-boat activity. 
Evasive routing had frustrated the normal German 
“hit and run” method of night attack on the convoys. 
However, it became clear in February that this had 
provoked intensified enemy offensive measures, in 
the form of greater cooperation between aircraft and 
U-boats, and special searching patrols. The days of 
wolf-pack attacks were foreshadowed as the U-boats 
started operating in groups of three to five, each 
U-boat being given a limited patrol area within the 
wider area covered by the group. The first U-boat to 
gain contact shadowed the convoy while others were 
ordered to concentrate in a position to attack. The 
shadower usually emitted radio signals to home other 
U-boats or aircraft to the attack. Similarly, aircraft 
were able to home U-boats to a convoy. 

Gooperation between U-boats, aircraft, and sur- 
face craft is well illustrated by the attack on HG 53, 
consisting of 21 ships escorted by one sloop and one 
destroyer. The convoy was attacked by a U-boat at 
0435 on February 9, two ships being sunk. The 
U-boat continued to shadow the convoy and prob- 
ably homed six Focke-Wulf aircraft to it during the 
afternoon of the 9th. Five ships were bombed and 


sunk while one plane was shot down. The U-boat 
continued to shadow the convoy and again attacked 
on the 10th, sinking one ship. After this, she main- 
tained touch with the convoy, reporting its position. 
Her reports were evidently intended for a German 
“Hipper” class cruiser. While closing HG 53, this 
cruiser came upon the unescorted slow portion of 
SL 64 and directed her attack against this easy target, 
sinking seven ships. 

Three other convoys were attacked by U-boats in 
the last week of February, and as the month drew to 
an end, with the losses mounting to 36 ships of 189,- 
000 gross tons, it was evident that the expected spring 
offensive of the U-boats had commenced. The aver- 
age number of U-boats at sea in the Atlantic rose to 
16 in March and these included some of Germany’s 
most skillful U-boat captains. Their tactics included 
a repetition of the concentrated night attacks upon 
convoys, and six convoys were attacked during the 
month. The upward trend of shipping lost by U-boat 
action reported in February was maintained during 
March with the total losses reaching 40 ships of 239,- 
000 gross tons. These losses were considerably less 
than those recorded during September and October 

1940, the last previous period of intense U-boat activ- 
ity, and were not considered unduly alarming consid- 
ering the fact that the number of U-boats at sea in 
March 1941 was more than twice as great as in the 
earlier period. 

More encouraging was the evidence of the in- 
creased efficiency of antisubmarine escorts and of the 
fact that U-boats which attacked adequately escorted 
convoys could be dealt with effectively. This evidence 
was clearly demonstrated by the loss to Germany, 
during March, of her three outstanding U-boat aces 
(Prien, Kretschmer, and Schepke), the top three 
U-boat captains in terms of tonnage sunk, each hav- 
ing more than 200,000 gross tons of shipping to his 
credit. 

Prien, commander of U-47, was the first to be lost 
as a result of his attack on Convoy OB 293 when he 
sank one ship shortly after midnight on March 8, 

1941. HMS Wolvexine, one of the escorts, sighted 
smoke about 20 minutes after the attack on the con- 
voy and subsequently made contact with the U-boat. 
The U-boat was attacked for over five hours, during 
which time there occurred a remarkable chase of the 
U-boat on the surface for over an hour, before it was 
finally considered sunk. There were no survivors but 


COUNTERMEASURES TO THE U-BOAT 


11 


prisoners of war from U-boats sunk subsequently 
have supplied information from which it is believed 
that the Wolverine’s attacks were made on U-47, 
commanded by Prien. Berlin subsequently admitted 
the loss of Prien. 

U-100 (Schepke) and U-99 (Kretschmer) were both 
sunk as a result of the attack on Convoy HX 112, dur- 
ing which five ships were sunk. U-100 located the 
convoy, which had been reported earlier by U-99, on 
the evening of March 16. Later she sighted a de- 
stroyer astern and dived. At 0137 on the 17th, HMS 
Walker obtained Asdic contact and attacked with 
depth charges. Three further depth-charge attacks 
were carried out by HMS Vanoc and HMS Walker 
before contact was lost at 0250. Meanwhile U-100, 
which had been considerably damaged by the depth 
charging, had surfaced. While the escorts were pre- 
paring to take station for an organized search, 
Vanoc’ s radar operator reported a contact 1000 yards 
away and U-100 was subsequently sighted on the sur- 
face and rammed by HMS Vanoc. Schepke, who was 
caught and crushed between the stove-in side of the 
bridge and the periscope, was carried down with the 
sinking U-boat. 

While HMS Vanoc was picking up survivors of 
U-100, HMS Walker obtained Asdic contact. Al- 
though it was considered unlikely that another 
U-boat would remain so close, the Asdic operator was 
so convinced that he had a submarine contact that 
HMS Walker fired six depth charges. This attack was 
extremely accurate and brought U-99 to the surface 
almost at once. Both destroyers opened fire and U-99 
was abandoned shortly afterward, with Kretschmer 
and other survivors taken as prisoners. Kretschmer 
had been quite successful up to that point as he 
claimed a record total of 86,000 tons of shipping sunk 
on this last cruise, while his total sinkings had 
reached 338,000 gross tons, more than any other 
U-boat captain. 

There were immediate indications that the enemy 
was severely shaken by the results of his attacks on 
adequately escorted convoys. The only subsequent 
attack during the month was made far west before 
the antisubmarine escort had joined, when three 
ships were sunk from HX 1J5 at 22° west longitude. 
Beside the effect on the tactics and operations of 
U-boats, the loss of three of Germany’s most skillful 
U-boat commanders must have had a profound effect 
on U-boat morale. 


2 2 COUNTERMEASURES TO THE 
U-BOAT 

2-2- 1 Convoys 

The high percentage of hits obtained by U-boats 
in night attacks made it necessary in November 1940 
to increase the distance apart of convoy columns 
from about 600 yards to about 1000 yards. The dis- 
tance between ships in the same column was about 
400 yards. In December 1940 the distance between 
columns during the daytime was reduced back to 
600 yards to increase protection against air attacks. 

To counter the heavy losses suffered as a result of 
the night attacks by surfaced U-boats in September 
and October 1940 the escorts were stationed in posi- 
tions down each wing at a greater distance from the 
convoy than before. In the event of an attack, they 
were instructed to proceed outward from the convoy 
for a distance of 10 miles at full speed, firing star shell 
to illuminate the area where the U-boat might be, in 
an attempt to sight her or force her to submerge, 
thereby improving the chances of Asdic detection. If 
contact was made, two escorts were to hunt the 
U-boat, while the remainder were to rejoin the 
convoy. 

Later, when radar-ecpipped escorts became avail- 
able, they were stationed one on each beam of the 
convoy, about 4 miles from it in order to avoid back 
echoes from the convoy on the radar set. The beam 
escorts were to steam on the same and opposite 
courses as the convoy, zigzagging as requisite for self- 
protection. Another method of sweeping, which was 
under trial in order to effect an economy in fuel, was 
for the escort to start a slow turn of 360° when in a 
position abeam of the leading ships, thus sweeping 
outwards and astern at about 1° per second, and on 
completion assuming station abeam of the rear ships. 
The remainder of the escorts were disposed as before, 
but were instructed to bear in mind that, at night, 
the rear wing positions were the most important and 
that, in the event of a U-boat attack, star shell searches 
were to be made in the rear of the convoy also. 

By the beginning of this period, in July 1940, the 
convoy system was fully established and most of the 
subsequent changes were made necessary by enemy 
activity. This may again be illustrated by continuing 
the history of the transatlantic HX convoys, the main 
line of supply to England. Besides the serious U-boat 


12 


NIGHT SURFACED ATTACKS ON CONVOYS 


threat, enemy air and surface craft activity also had 
their effects on these convoys. 

The first HX convoy to be routed around the 
north of England in July 1940 made a rendezvous 
with the local antisubmarine escort at about 17° west 
longitude. Slow convoys to include ships between 
7 1/4 and 9 knots were organized in August to assemble 
at Sydney, Nova Scotia, and were designated SC. This 
reduced the HX convoys to a reasonable size of about 
45 ships. The HX convoys sailed on a 4-day cycle, 
while the SC convoys were on an 8-day cycle. In De- 
cember, Sydney was abandoned as a convoy assembly 
port, but the SC convoys were to continue to sail 
from Halifax. 

In order to extend the antisubmarine escort fur- 
ther west, the convoy intervals were lengthened, with 
HX convoys soling at alternate 6- and 4-day inter- 
vals, while the^C convoys sailed at 10-day intervals. 
In addition. Loch Ewe, in the northern part of Scot- 
land, was started as an assembly port for ships on the 
east coast of England, and destroyers serving as anti- 
submarine escorts were able to refuel there and oper- 
ate further west. 

In Eebruary 1941, two HX convoys were routed on 
a southern course but heavy air attacks resulted in 
these convoys being rerouted to the north. Following 
an attack by a surface raider and the sighting of two 
German battle cruisers in the Atlantic, it was decided 
to give close battleship cover to all Halifax convoys. 
This threat of surface raiders also led to the discon- 
tinuance of the Bermuda section of HX convoys and 
ships were routed independently to the Halifax 
assembly. 

British convoys were much harder hit during this 
second period than in the previous period. The num- 
ber of ships convoyed monthly increased to about 
3600, with 26 of these being sunk monthly by 
U-boats (13 in escorted convoys, 3 in unescorted con- 
voys, and 10 stragglers). This meant that the total 
loss rate to U-boats was about 0.7 per cent, more than 
three times as high as in the earlier period. 

Moreover, the HX and SC convoys sailing across 
the Atlantic to England were much harder hit than 
other convoys. Of the 360 ships sailing monthly in 
these convoys (only 10 per cent of the total convoyed 
shipping), about 14 ships were sunk monthly by 
U-boats (over 50 per cent of the total losses of con- 
voyed shipping). The loss rate to U-boats on these 
vital convoys was about 4 per cent, more than five 
times as high as for all convoys. 


Despite these high losses in convoys, the chances 
of being sunk by a U-boat were still higher for inde- 
pendently routed ships. Comparable figures are avail- 
able for shipping passing through the Northwestern 
Approaches during the 6-month period from Sep- 
tember 1940 through February 1941. About 80 per 
cent of the total losses to U-boats occurred in that 
area. Of the 1180 ships sailing through this area 
monthly in convoy, about 29 were sunk monthly by 
U-boats for a loss rate of about 21/4 per cent. Of the 
70 independently routed ships sailing through this 
area monthly, about three were sunk monthly by 
U-boats for a loss rate of about 4 per cent. In making 
this comparison, one should keep in mind that the 
ships sailing independently were generally capable 
of making a speed of at least 13 knots, much higher 
than the average speed which ships sailing in con- 
voys were capable of making. This means that if all 
shipping had sailed independently the loss rate 
would probably have been much higher than the 
4 per cent experienced by the select group of ships 
that sailed independently. 

Aircraft 

During the second period, increased use was being 
made of aircraft as a counter to U-boats. Though 
suffering from many limitations for antisubmarine 
operations, the airplane possesses obvious advantages 
denied to surface craft (e.g., speed, cheapness, large 
field of vision, and economy of personnel and ma- 
teriel). A U-boat on the surface can rely on the fact 
that she will almost certainly sight an enemy ship 
before she, herself, is seen. However, she must always 
keep a vigilant lookout against being surprised by a 
plane sweeping down out of the clouds. 

In an attempt to improve the lethality of aircraft 
attacks. Coastal Command tried using naval depth 
charges modified for air use. Sunderland aircraft 
started carrying both depth charges and bombs in 
July 1940. The first success of this new weapon came 
on August 16 when a U-boat was severely damaged as 
a result of a depth-charge attack. The Sunderland 
plane, carrying two depth charges and four 250- 
pound antisubmarine bombs, first dropped a single 
depth charge set to explode at a depth of 100 feet 
about 20 yards ahead of the conning tower of the 
submerging U-boat. The U-boat was forced to the 
surface and two minutes later the second depth 
charge, set for 150 feet, was dropped about 20 feet 


C 


lL 


COUNTERMEASURES TO THE U-BOAT 


13 


ahead of the conning tower. The U-boat was again 
blown to the surface and was then observed to sink 
sideways. On the third attack the stick of four bombs 
was dropped on the submerged U-boat. Air and oil 
came to the surface. In view of the initial successes 
of depth charges, steps were immediately taken to 
modify other Coastal Command aircraft in order to 
enable them also to carry depth charges. It was ex- 
pected that the lethal value of aircraft attacks on 
U-boats would be considerably increased by this 
change. 

The night attacks on convoys led, in September 
1940, to the fitting of radar to the aircraft of Coastal 
Command and the Fleet Air Arm. This was supposed 
to be especially valuable for detecting U-boats on the 
surface at night and it was hoped that this would 
make it possible to operate aircraft at night for con- 
voy escort work. It was also intended to provide the 
maximum air escort for the three hours before dark- 
ness falls, as this is the period in which U-boats could 
be found in shadowing positions preparatory to the 
night attack. 

After the evasive routing of shipping had led to the 
start of wolf-pack tactics in February 1941, the 
shadowing U-boat became the main problem. Hav- 
ing contacted a convoy, the U-boat took great care 
not to reveal her presence by attacking in daylight, 
but shadowed the convoy at some distance. There 
was, therefore, only a small chance of the limited 
number of escorts discovering these U-boats and this 
task fell to the escorting aircraft. In view of this it 
was decided to reinforce the number of Coastal Com- 
mand aircraft available for escort duty in the North- 
western Approaches. Consideration was also given to 
the problem of evolving the best type of aircraft 
patrol, round the convoy, to prevent the U-boat from 
shadowing it. 

Despite the curtailment of routine antisubmarine 
patrols in favor of anti-invasion patrols during this 
period, the average number of hours flown monthly 
by Coastal Command aircraft on antisubmarine 
duties increased by about 1000 hours over the pre- 
vious period to reach 6300 hours, 5100 hours on con- 
voy escort and 1200 hours on patrol. The number of 
flying hours on antisubmarine work dropped to 
about 4000 during the winter months of December 
1940 and January 1941, due to longer hours of dark- 
ness and poorer weather. By March 1941 it was again 
up to about 8000 hours. This increased amount of 
flying was less productive than during the first period. 


as the number of sightings made monthly dropped 
to about 14 and the attacks to about 8. This decrease 
in the number of sightings, despite the increased 
number of U-boats at. sea, was due mainly to the 
movement of the U-boats further westward, out of 
range of much of the flying. Again, about 10 per cent 
of the attacks resulted in some damage to the U-boat 
but the lethality of the attacks improved, as two 
(about 21/2 per cent) of the attacks resulted in the 
probabl e sinking of a U-boat. 

Scientific and Technical 

Consi derable research was done during this period 
on improving Asdic sets with one of the chief goals 
being the development of practical depth-determin- 
ing gear. Very little progress was made on this diffi- 
cult problem and the only immediate solution was 
the use of larger depth-charge patterns to counter- 
balance the large effect of the unknown factor of 
depth. 

Another scientific development during this period 
was the extensive use of high frequency - direction 
finding [HF/DF] towards the end of 1940, after Ger- 
many had acquired the French bases and the U-boats 
had started widespread operations in the Atlantic. 
The principle of HF/DF was that a shore station 
could determine the bearing of any U-boat making 
a radio transmission, and it was hoped that the point 
of interception of the bearings from several shore 
stations would determine the transmitting U-boat’s 
position. However, as more HF/DF shore stations 
became available around the Atlantic shores and as 
U-boats started to operate in numbers on the Atlan- 
tic trade routes, it became clear that shore-based 
HF/DF could only provide a rough indication of the 
general area in which the U-boat was and, at best, it 
could only provide a warning for a threatened con- 
voy and so assist convoy routing. The Germans ap- 
preciated this and felt that shipborne direction-find- 
ing was restricted to medium frequencies. They 
therefore used high-frequency communications ex- 
tensively once contact had been made with the con- 
voy. As a result it was realized that HF/DF on convoy 
escorts themselves might do a great deal more; it 
might even enable the escorts to find U-boats before 
they could launch their attacks. The immediate re- 
quirement was an HF/DF outfit for ships which was 
quick and easy to operate. 

However, the main scientific achievement during 
this period was the introduction of radar sets on 


14 


NIGHT SURFACED ATTACKS ON CONVOYS 


both ships and aircraft. Radar worked on the prin- 
ciple of transmitting short pnlses of very high-fre- 
quency radio waves and then receiving the echoes 
from objects, like a U-boat on^the surface. The echoes 
would enable the range and bearing of the object to 
be determined even at night and in conditions of 
poor visibility. 

We have seen that the heavy shipping losses suf- 
fered at the start of this period as a result of night 
attacks on convoys had made radar an urgent neces- 
sity. As a stop-gap, the first radar sets fitted in de- 
stroyers were of a Royal Air Force design known as 
air-surface vessel [ASV] or in the British Navy as 
radio direction-hnding [RDF] Type 286 M. The fit- 
ting of these sets on ships was started about Novem- 
ber 1940 and by April 1941 radar had been fitted on 
about 40 destroyers of the Western Approaches Com- 
mand. It was hoped that radar would enable the 
escorts to detect the presence of any U-boat on the 
surface within a radius of some two or three miles. 

Type 286 M had a fixed aerial and received echoes 
from a target over an arc covering about 50 degrees 
on each side of the bow and also over a similar arc 
astern at considerably shorter ranges (back echoes). 
The wavelength of these early sets was relatively 
long, over a meter, and consequently the aerial had 
to be very high above the surface of the sea before 
any considerable range could be obtained on small 
objects. This limited the effectiveness against U-boats 
of early radar sets on ships, but not on aircraft, as a 
plane flying at 2500 feet could expect to detect a 
U-boat on the surface at a range of about 15 miles. 

Radar could be used in antisubmarine warfare for 
several subsidiary purposes, besides the main one of 
detecting U-boats. It could give warning of the ap- 
proach of aircraft; it could be used in low visibility 
to make contact with single merchant ships or con- 
voys; to pick up navigation buoys; to keep station on 
a convoy at night; or for making landfall. • 

By January 1941 it appeared that, as an antisub- 
marine device, radar on surface ships had been a dis- 
appointment. Escorts had considerable trouble ow- 
ing to confusing “back echoes” from the convoy. As a 
temporary measure it was hoped to alleviate this 
trouble by reducing the range scale from ten miles 
to five miles. Work was also being done on a newly 
designed aerial, screened to cut out back echoes. At 
the end of this period, in March 1941, new types 
using shorter wavelengths and directional aerials 
were under trial and an improved radar set of naval 


design, Type 290, was in production and was to 
replace Type 286. 

Sinking of U-boats 

Surface craft continued to be the most effective 
craft in attacking and sinking U-boats during this 
period, making about 25 attacks a month. Of the 23 
U-boats sunk or probably sunk in the Atlantic, sur- 
face craft could be credited with 13, or about 53 per 
cent. Submarines proved highly effective early in this 
period, patrolling close to the French bases, and tor- 
pedoing five U-boats in September 1940 and another 
in December. These submarine attacks made it neces- 
sary for the U-boats to enter and leave their bases 
submerged. Two U-boats were probably sunk as a 
result of aircraft attacks, one was sunk as a result of 
a combined attack by a ship and plane, and another 
was mined. 

In addition, one German U-boat was known to 
have been sunk under unknown circumstances while 
1 1 Italian U-boats were sunk in the Mediterranean, 
with surface craft again accounting for six, or 55 per 
cent of them. Italian U-boats operating outside the 
Atlantic had very little success against Allied ship- 
ping, as only five ships of 28,000 gross tons were sunk 
in the Mediterranean and Indian Oceans during the 
first two years of the war, from September 1939 to 
September 1941. 

2 3 SURVEY OF RESULTS 

2.3.1 From the U-boat’s Point of View 

The new U-boat tactics adopted during this second 
period had accomplished their primary objective of 
reducing the high rate of loss of U-boats. The average 
number of U-boats at sea in the Atlantic during this 
period rose to about 10, while only about 21/9 of these 
were lost monthly. The average life of a U-boat at 
sea had increased by about 33 per cent, from 3 
months to 4 months. 

In addition, the efficiency of U-boats in sinking 
ships increased slightly, as the average U-boat sank 
four ships of about 22,000 gross tons per month at 
sea. The combined effect of these two factors im- 
proved the overall exchange rate to 16 ships of about 
88,000 gross tons sunk for each U-boat sunk or prob- 
ably sunk, an extremely profitable transaction for 
the U-boats. 


SURVEY OF RESULTS 


15 


From a quantitative point of view, the position of 
the German U-boats had improved considerably dur- 
ing this period as the increased U-boat building pro- 
gram, which the Germans had started shortly after 
England entered the war, began to take“effect. As a 
result of commissioning about 45 new ocean-going 
U-boats while losing only about 18 of the large ones 
(500 tons or larger) Germany had about 54 ocean- 
going U-boats available at the end of this period, 
about twice as many as it had at the start of the 
period. 

Thus the Germans would be able to send many 
more U-boats out to sea during the third period than 
they had sent during the early periods of the war. 
However, they had lost many of their ablest and most 
experienced captains and crews, and these were not 
as .easily replaced as the U-boats themselves. The 
necessity of sending out relatively inexperienced 
U-boat captains was probably a factor influencing the 
Germans to decide, in February 1941, to operate their 
U-boats in groups, so that several inexperienced cap- 
tains could operate together with a more experienced 
one. 

2.3.2 From the Allies’ Point of View 

Total shipping losses of the Allied and neutral 
nations were about 456,000 gross tons a month dur- 
ing the second period, more than 60 per cent higher 
than during the first period. Meanwhile the building 
rate had increased only slightly to about 114,000 
g^oss tons a month, making the net loss of shipping 
about 342,000 gross tons a month. Total shipping 
available had decreased from about 38,000,000 gross 
tons at the start of the second period to about 35,000,- 
000 gross tons at the end of the second period. 

Of the 456,000 gross tons of shipping lost monthly, 
about 404,000 gross tons were lost by enemy action. 
U-boats accounted for 42 ships of 224,000 gross tons 
a month (55 per cent of the total tonnage lost by 


enemy action), more than twice the monthly tonnage 
sunk by U-boats during the first period. Monthly 
shipping losses due to enemy surface craft jumped to 
87,000 grpss tons (22 per cent) and those due to enemy 
aircraft increased to 61,000 gross tons (15 per cent). 
Monthly losses due to mines dropped from second 
place in the first period to only 27,000 gross tons (7 
pel cent), with other and unknown causes accounting 
for the other 1 per cent of the total losses due to 
enemy action. 

There is no doubt that the U-boats had inflicted a 
serious defeat on the Allies in the Battle of the Atlan- 
tic during the second period, but the situation was 
beginning to look more promising toward the end 
of this period. One favorable element was the increas- 
ing number of antisubmarine ships and aircraft be- 
coming available for convoy escorts as the threat of 
the invasion of England was decreasing. The number 
of antisubmarine ships suitable for ocean escort {i.e., 
destroyers and patrol craft such as sloops, frigates, 
corvettes) had increased from about 235 at the start 
of this period to about 375 (includes 240 destroyers) 
at the end of the period. Important factors in this 
increase were the coming into service of the new 
corvette and also the transfer of the 50 old Town 
class destroyers from the United States to England 
in September 1940. These destroyers were equipped 
with U. S. echo-ranging gear, called sonar, which was 
similar in principle to the British Asdic. 

In addition, an increasing number of ships and 
planes were being equipped with radar in order to 
combat the U-boat s night activity. Officers and crews 
had increased experience and training in antisub- 
marine warfare and had shown in March 1941 that 
they could inflict heavy losses on U-boats attacking 
adequately escorted convoys. It appeared as if the 
main problem during the third period would be that 
of meeting the westward movement of the U-boats by 
extending antisubmarine escort westward, without 
weakening the escort strength. 


Chapter 3 

THIRD PERIOD 

START OF WOLF PACKS; END-TO-END ESCORT OF CONVOYS 

APRIL 1941-DECEMBER 1941 


S I U-BOAT OFFENSIVE 

T he FRUITS of the intensified German U-boat con- 
struction program, started late in 1939, were be- 
ginning to appear as the average number of U-boats 
at sea in the Atlantic steadily increased from about 
18 in April 1941 to about 36 in August 1941. The 
main features of U-boat tactics during this third 
period were the increasing use of wolf-pack attacks 
forced upon the Germans by the evasive routing of 
British convoys and the scarcity of experienced 
U-boat commanders. 

The outstanding successes achieved by escorts in 
the Northwestern Approaches during March 1941 
produced the direct result that, in April, U-boats 
abandoned the method of close attack on the surface 
while antisubmarine escorts were in company. The 
U-boats continued their search for weak spots in the 
antisubmarine defenses by moving away from the 
vicinity of England, where air coverage was heavy, 
and extended their operations further westward 
where they could attack convoys before the anti- 
submarine escort had joined. There was also a south- 
ward movement of the U-boats with increased activ- 
ity in the Azores and Freetown Areas. 

April opened somewhat disastrously with heavy 
attacks, started before the antisubmarine escort had 
joined, on Convoy SC 26. About five U-boats partici- 
pated in these attacks, ten ships were sunk, and, in 
addition, the armed merchant cruiser ocean escort 
was damaged by a torpedo hit. One of the attacking 
U-boats was sunk after the antisubmarine escorts had 
joined. Towards the end of April, four ships were 
sunk from Convoy HX 121 as a result of the first 
submerged daylight attack by a pack of U-boats. The 
shipping losses to U-boats in April were about the 
same as in March, with 41 ships of 240,000 gross tons 
sunk. However, only about 30 per cent of the ton- 
nage sunk by U-boats was in convoy in April as com- 
pared with 60 per cent in March. About 13 per cent 
of the tonnage sunk by Urboats in April was sunk in 


the Azores Area and another 15 per cent in the 
Freetown Area. 

The total shipping losses, from all causes, 
amounted to 682,000 gross tons in April 1941, a 
higher figure than for any previous month in the war. 
This was due mainly to the heavy shipping losses to 
enemy aircraft, about 296,000 gross tons, most of 
these losses occurring in the Mediterranean in con- 
nection with the evacuation from Greece and Crete. 

As a result of the heavy attack on Convoy SC 26 at 
about 28° west longitude, the Iceland routing scheme 
was adopted earlier than was originally intended. 
Escorts were based on Iceland, making it possible to 
meet convoys where the escort from England had to 
leave, and then to escort the convoys out to about 35° 
west longitude, the escort there picking up an incom- 
ing convoy and then turning it over to an escort 
group from England. Sunderland and Hudson air- 
craft were also moved to Iceland to provide air cov- 
erage for convoys in waters which could not be 
covered by aircraft based on England. 

Obviously this considerable increase in the dis- 
tance over which transatlantic convoys were escorted 
was only achieved at the expense of weaker individ- 
ual escorts with each convoy. This was partly com- 
pensated by reinforcing the Western Approaches 
with Asdic-fitted minesweepers. It is equally clear 
that the use of Iceland necessitated a certain rigidity 
of routing and tended to make the location of con- 
voys by U-boats and enemy aircraft a simpler busi- 
ness. Against this, the daylight hours in these north- 
ern latitudes were rapidly lengthening as the sum- 
mer months approached and the U-boat danger in 
daylight was a lesser menace than at night. 

The upward trend in shipping losses to U-boats 
continued as 58 ships of 325,000 gross tons were sunk 
in May. Over half the losses occurred in the Free- 
town Area, where a group of about six U-boats sank 
32 ships of 186,000 gross tons during the month. To 
meet the increased U-boat activity in that area, addi- 
tional escorts were added to the Freetown forces and 




16 


u 


U-BOAT OFFENSIVE 


17 


action was taken to divert all shipping from the 
Freetown Area, except those ships which must of 
necessity pass through those waters. 

Attacks on independents continued to increase as 
only about 20 per cent of the shipping sunk by 
U-boats in May was in convoy. In addition, the 
U-boats continued to move further westward and on 
May 20 located Convoy HX 126 at about 41° west 
longitude, before the antisubmarine escorts had 
joined. Eight ships were sunk before the convoy was 
forced to disperse. This attack forced the adoption of 
complete transatlantic escort. 

It was felt that the considerable weakening in the 
number of escorts with a convoy must be accepted in 
order to provide some degree of protection through- 
out the voyage. Complete transatlantic escort was 
accomplished by basing escort forces in St. John’s, 
Newfoundland, and escorting in stages from Eng- 
land, using Iceland as a refueling base. The Royal 
Canadian Navy cooperated in these measures by plac- 
ing all available destroyers and corvettes at the serv- 
ice of the Newfoundland Escort Force. Canada had 
about 35 ships fitted for antisubmarine service at 
that time. The first escorts from St. John’s sailed on 
May 31, 1941, and, as a natural sequence to this de- 
velopment, it was decided by the middle of June to 
escort the convoys all the way from Halifax. The 
long-endurance corvettes were to run all the way be- 
tween Halifax and Iceland, with the destroyers being 
limited to St. John’s. 

Areas of U-boat activity in June were further afield 
and wider spread than before, with reports of U-boats 
near Newfoundland and south of Greenland. De- 
spite the magnitude of the effort exerted, the ship- 
ping losses showed an improvement over May, with 
57 ships of 296,000 gross tons being sunk by U-boats 
in June. Despite the increasing number of U-boats 
at sea, the losses were kept down by the efficiency of 
British countermeasures as five U-boats were sunk 
during June by surface craft. Losses in the Freetown 
Area were greatly reduced and the U-boats had diffi- 
culty in locating the transatlantic convoys. 

When they did finally locate Convoy HX 133 on 
June 23, the results must have been rather disap- 
pointing to the Germans, as only five ships were sunk 
at the cost of at least two U-boats sunk. This success- 
ful defense of this convoy was due in large measure 
to the fact that, when DF bearings indicated that HX 
133 had been sighted by a U-boat, the escort was in- 
creased from one destroyer and three corvettes to two 


destroyers, one sloop, and ten corvettes. This was 
accomplished by taking the risk of stripping the es- 
corts from two OB convoys within comparatively 
easy reach. Fortunately, one of these OB convoys 
escaped unscathed while the other suffered the loss 
of only one ship. 

On June 22, 1941, Germany invaded Russia and 
this seemed to end the threat of invasion of England 
for the time being. This released additional air and 
surface craft to help in the battle against the U-boats. 
In addition, German aircraft were diverted to the 
Eastern Front and attacks on shipping by aircraft 
were greatly reduced during the last half of 1941. 

The average number of U-boats at sea continued 
to increase during July and August, but they had 
very little success as only about 23 ships of 90,000 
gross tons were sunk in each of these months. This 
meant that the average U-boat at sea in the Atlantic 
was sinking less than one ship a month, a much lower 
rate than had been experienced in the past. In an 
endeavor to make the interception of shipping easier 
the U-boats withdrew to the eastward towards the 
end of July and concentrated in the waters west of 
Ireland and to the east of 25° west longitude. This 
placed them at a focal point of shipping where they 
could intercept both the East-West and the North- 
South convoys. However, the U-boats had no better 
luck there in August than they had in July. By this 
time, even the Gibraltar and Freetown convoys had 
more or less complete end-to-end escort. Towards the 
end of August, there were indications that the 
U-boats were resorting to long-range attacks on con- 
voys, probably firing a browning salvo, and also to 
deliberate attacks on escorts. This policy of attack- 
ing escorts might have proved more prohtable in the 
earlier days of the war, when the number of escorts 
with convoys was much smaller and when the general 
escort situation was much tighter. 

In addition to the defensive successes scored in 
August, this month was marked by one of the out- 
standing events of the U-boat war, the surrender of 
U-570 to a Hudson aircraft on August 27, 1941. U-570 
left on her first cruise on August 24 and was at sea for 
only 74 hours before she surrendered. The U-boat 
came to the surface at 1030 on the 27th, the precise 
moment the Hudson from Squadron 269 was over- 
head. The U-boat tried to crash dive but the Hudson 
was too quick for her, diving from 500 feet to 100 
feet, and dropping four depth charges. Captain 
Rahmlow, believing the U-boat more seriously dam- 


18 


START OF WOLF PACKS; END-TO-END OF CONVOYS 



Figurf, 1. Simcnder of lJ-570 (HMS Graph) to Hudson aircraft S-269 on August 27, 1941, Note German crew crowded 
into conning tower. 


aged than was actually the case, ordered the crew to 
put on life jackets and go to the conning tower. The 
Hudson opened fire and kept the crew from aban- 
doning the U-boat. After a white flag was displayed 
by the U-boat the Hudson guarded it until relieved 
by a Catalina. A trawler arrived at 2250 and U-570 
was towed to Iceland. She was subsequently repaired 
and re-christened HMS Graph, proving an invalu- 
able addition to the British Navy. This was the first 
U-boat actually captured by the British and proved 
to be an extremely valuable source of information 
about the operating characteristics of U-boats. 

U-boat activity continued at a high level in Sep- 
tember and their intensive efforts met with greater 
success than during the previous two months. Allied 
shipping losses rose sharply to 54 ships of 205,000 
gross tons sunk by U-boats, with 70 per cent of the 
tonnage sunk being in convoy. This increase was due 
in a large measure to the severe casualties suffered by 


four large convoys as a result of determined and sus- 
tained attacks by wolf packs. Two of these convoys 
were slow SC convoys which were intercepted and 
heavily attacked south of Greenland, losing 21 ships 
and one escort. Two U-boats were sunk by escorts 
during these attacks. The other two convoys, home- 
ward bound from Freetown and Gibraltar, lost 15 
ships and one escort to the U-boats. 

However, in viewing the situation at this time it 
would be well to compare it with the previous year. 
In September 1940, when about seven U-boats were 
at sea, the losses to U-boats were about 300,000 gross 
tons. In September 1941, when there were about 35 
U-boats at sea, the losses to U-boats were only about 
200,000 gross tons. In September 1940 the U-boats 
were attacking convoys with impunity. Rarely was a 
U-boat sighted during her attack, and even more 
rarely was she counterattacked. In contrast to this, in 
September 1941 it was a matter of the keenest disap- 



U-BOAT OFFENSIVE 


19 


pointment if a convoy were attacked and the enemy 
escaped unpunished. The limited number of escorts, 
usually about one destroyer and three or four cor- 
vettes, although unable to prevent the U-boats from 
attacking the convoy, were generally able, with the 
help of covering aircraft, to shake off the pursuing 
U-boats by persistent counterattacks. No convoy was 
attacked for more than three successive nights in 
September 1941. 

The effect of Coastal Command aircraft on U-boat 
operations may be seen by the fact that, of the ton- 
nage sunk in the North Atlantic during September 
by U-boats, about 75 per cent was lost in the area 
outside the economical range of Whitley and Wel- 
lington aircraft (400 miles). Aircraft made 45 sight- 
ings and 39 attacks on U-boats in September, the 
highest monthly figures recorded to that date. 
September was also marked by the introduction 
of HMS Audacity, an auxiliary aircraft carrier, as a 
convoy escort. One of her fighter aircraft shot down 
a Focke-Wulf attacking the rescue ship of Convoy 
OG 74. 

As the radius of U-boat operations in the Atlantic 
extended further west and U. S. ships were being 
sunk by U-boats, the U. S. Navy announced on Sep- 
tember 15, 1941, that it would provide protection for 
ships of every flag carrying land-aid supplies between 
the American continent and the waters adjacent to 
Iceland, on which a U. S. base had been established 
in July 1941. On September 16 the first convoy (HX 
150) to have U. S. Navy ships as part of its escort 
sailed from Halifax. 

The losses in October 1941 dropped to 32 ships of 
157,000 gross tons sunk by U-boats. Only one convoy 
(SC 48) was heavily attacked by U-boats; nine ships 
and two escorts were sunk and the USS Kearny was 
torpedoed but arrived at Iceland. The USS Reuben 
James was sunk by a U-boat torpedo on October 31 
while acting as an escort of Convoy HX 156. In a 
number of other cases the convoys were located by 
U-boats but the escorts were able to drive them off 
without suffering serious losses. 

An interesting feature of the operations during 
October was the disinclination of the U-boats to pur- 
sue their quarry too far northward or eastward, pre- 
sumably because they did not care to enter the areas 
swept by Coastal Command aircraft. This is indi- 
cated by the fact that of the 26 ships sunk within 800 
miles from air bases, 14 were lost in portions more 
than 600 miles out and 12 in the 400- to 600-mile 


zone (covered lightly by Catalinas). No ships were 
sunk within 400 miles from Coastal Command bases. 
This meant that the U-boats were being forced ever 
further westward, with consequent greater wastage of 
time and U-boats, particularly in winter. In addition, 
air attacks on transit U-boats in the Bay of Biscay 
began to increase in frequency and effectiveness, 
thereby further cutting down the operational time 
of U-boats. 

During the first week of November, the scale of the 
U-boat effort was probably the greatest and the scope 
of their patrol the widest spread of the whole Atlan- 
tic campaign up to that time. When Convoy SC 52 
was intercepted shortly after rounding Newfound- 
land and four ships were sunk on November 3, it was 
considered prudent not to risk it upon a transocean 
journey for the greater part of which many U-boats 
might have maintained continuous harrying attacks. 
The convoy put back to port and the ships sailed 
later in Convoy SC 54, which consisted of 71 ships. 
This decision proved fortunate, as there followed a 
period of successful evasion which lasted for the re- 
mainder of the month. The total losses for November 
1941 dropped to 12 ships of 62,000 gross tons, the 
lowest figure since May 1940. Weather contributed to 
the reduction in shipping losses but the main factor 
was skillful evasive routing. Successful evasion meant 
fewer chances of contacts between escorts and 
U-boats and therefore less chance for the destruction 
of U-boats. 

In the meantime the British offensive in Libya had 
been launched and the Germans withdrew a large 
proportion of their U-boats from the Atlantic to the 
Mediterranean to help the Italian Fleet and tem- 
porarily, at least, to use their U-boats for the trans- 
port of military supplies to Rommel. Toward the end 
of November, convoys from Gibraltar were sus- 
pended and every available ship was used in an en- 
deavor to close the Straits of Gibraltar to the passage 
of U-boats and to destroy U-boats attempting pas- 
sage. Two German U-boats attempting the passage 
were sunk during the month, one by surface craft 
and the other by a Dutch submarine. 

The perceptible slackening in tension in the North 
Atlantic that started toward the end of November 
continued throughout December 1941. The losses in 
the Atlantic continued at a very low level, with only 
10 ships of about 50,000 gross tons sunk by U-boats 
during the month. However, the tendency of the 
U-boat war to become world wide became apparent. 


20 


START OF WOLF PACKS; END-TO-END OF CONVOYS 


as for the first time the losses to U-boats outside the 
Atlantic became significant. Seven ships of 27,000 
gross tons were sunk by U-boats in the Mediterra- 
nean, three in the eastern approaches to Gibraltar 
and four while on passage between Alexandria and 
I'obruk. Howe\er, the enemy paid a heavy price for 
these seven ships, as five U-boats were sunk in the 
Afediterranean during December. 

Japanese submarines sank nine ships of 42,000 
gross tons in the Pacific during the remainder of De- 
cember after their attack on Pearl Harbor on De- 
cember 7, 1941. One Japanese submarine was prob- 
ably sunk. Over 200,000 gross tons of shipping were 
lost in the Pacific due to capture by the Japanese or 
unknown causes. Accordingly, the total shipping 
losses for the month (from all causes) rose to over 
500,000 gross tons. 

The outstanding feature of the month’s operations 
in the Atlantic was the prolonged engagement be- 
tween Convoy HG 76 and a pack of half a dozen 
U-boats. The escorts sank four U-boats (including 
U-567, commanded by Endrass, one of the leading 
U-boat aces) and only two of the merchant ships of 
the convoy were lost. However, one escort and HMS 
Audacity, an auxiliary aircraft carrier, were also sunk 
by the U-boats. As 1941 drew to an end, the number 
of U-boats in the Atlantic began to rise again, the 
majority leading westward. 

3 2 COUNTERMEASURES TO THE 

U-BOAT 

Convoys 

U-boat activity during the early months of this 
period forced the adoption of complete end-to-end 
escort of British convoys. Thanks to the steadily in- 
creasing number of escorts becoming available, and 
to cooperation from the Canadian and United States 
Navies, this did not result in the anticipated serious 
weakening in the protection of the convoys. In fact, 
ships sailing in convoy were safer in this period than 
they had been in the preceding period, as of the 4100 
ships being convoyed monthly only 14 (about 
of 1 per cent) were sunk monthly by U-boats. By the 
end of 1941, it appeared, as evidenced by the battle 
of Convoy HG 76, that the wolf-pack attacks on con- 
voys could be beaten off without serious losses to the 
convoy and in turn these attacks could prove costly 
to the U-boats, provided a sufficient number of es- 
corts were present. 


An analysis of 17 of the convoys attacked by the 
U-boats at about this time gives us some information 
about the “average attacked convoy.” This convoy 
was engaged by 4.2 U-boats, of which 2.6 succeeded 
in delivering effective attacks. A total of 4.6 ships in 
the convoy were torpedoed, 1.7 ships being torpedoed 
in each effective attack. Of the 4.2 U-boats engaging 
the convoy 3.2 (or 76 per cent) were attacked by the 
air and surface escorts and 0.65 (or 15 per cent) of 
them sunk. 

There were not many changes made in the convoy 
system during this period. The minimum speed for 
HX convoys was raised from 9 knots to 10 knots, with 
the minimum speed for SC convoys remaining at 7i/2 
knots. In July 1941 a rule was introduced requiring 
ships of less than 15 knots, crossing the Atlantic, to 
sail in convoy. The designation of the OB convoys 
(outward-bound from England) was changed to ON 
for the north-bound convoy heading for Halifax and 
to OS for the south-bound convoys heading for Free- 
town. July 1941 also marked the introduction of 
CAM ships in convoy. These were merchant ships 
equipped with a catapult-launched fighter aircraft 
which was to be used against enemy reconnaissance 
aircraft. In August 1941, the first convoy to Russia 
sailed for Archangel. No losses were suffered on the 
convoys sailing between England and Russia during 
1941. 

Aircraft 

At the beginning of this period, in April 1941, it 
was apparent that an improvement in the quality of 
aircraft attacks was urgently needed. Actual kills by 
aircraft had been disappointingly few and it was felt 
that, on the relatively rare occasions when a pilot 
sights a U-boat, he should have a reasonably good 
chance of bringing off a kill. A committee of Coastal 
Command scientists and naval representatives was 
therefore formed to review this situation. 

Analysis showed that in 35 per cent of the aircraft 
attacks the U-boat was still visible at the time of re- 
lease of the depth charges and that in 15 per cent of 
the attacks the U-boat had disappeared less than 30 
seconds previously. The solution adopted was to con- 
centrate on those U-boats which were still on or near 
the surface and to adjust tactics, deptii-bomb settings 
and spacing, so as to give the aircraft the maximum 
chance of killing in these conditions, since the prob- 
ability of hitting the U-boat under these conditions 
is much greater than it is after the U-boat has been 
submerged for some time. 


COUNTERMEASURES TO THE U-BOAT 


21 


Instructions were accordingly issued by Coastal 
Command in June 1941 providing that the depth 
setting for all depth charges should be 50 feet, the 
spacing between charges should be 60 feet, and all 
depth charges were to be released in one stick. The 
need for more frequent practice and training was also 
stressed and a standard for bombing accuracy was set 
up requiring a mean radial error of all bombs 
dropped of not more than 20 yards. 

This change resulted in a significant improvement 
in the quality of aircraft attacks on U-boats. Aircraft 
made about 27 sightings a month during this period 
and about 18 of these resulted in attacks on U-boats. 
The proportion of these attacks which resulted in at 
least some damage to the U-boat rose from about 10 
per cent in the earlier periods to about 25 per cent 
in this period. The lethality of the aircraft attacks 
did not change much, as only about 21/2 per cent of 
the attacks resulted in the U-boat’s being sunk. How- 
ever, it was realized at the end of this period that 
even the 50-foot depth setting was too deep for sur- 
faced U-boats and steps were being taken to modify 
depth charges to allow a 25-foot depth setting. 

As the U-boats moved further westward, the total 
number of flying hours by Coastal Command air- 
craft on antisubmarine duties decreased, but transit 
U-boats crossing the Bay of Biscay on their way to 
and from French bases began to receive more atten- 
tion. In the period September to November 1941, 36 
attacks were made on U-boats in this area. This was 
equivalent to each U-boat’s being attacked in the 
Bay on one out of every three cruises and was a suffi- 
cient menace to force them to remain submerged in 
the Bay during the daytime in December, thereby 
increasing their transit time. By December 1941, 
Coastal Command had initiated night antisubmarine 
patrols with radar-fitted aircraft in the Bay of Biscay. 

Midway in 1941, the British scraped together a 
squadron of long-range planes, new untried Liber- 
ators rejected by U. S. Air Forces. Fitted with Mark 
II radar, they did a dual job, ranging far out on 
patrol and giving air cover to convoys asking for help. 

The flying effort during this period was marked 
by the increased use of protective sweeps around con- 
voys to put down shadowing U-boats, as distinct from 
the close convoy coverage provided earlier. It was 
also marked by the first attempt, in May 1941, by 
radar-equipped aircraft to hunt a U-boat to exhaus- 
tion. Although this attempt did not succeed, much 
useful information was obtained from this operation. 


Scientific and Technical 

A new antisubmarine weapon for use by surface 
craft was coming into production at the end of 1941. 
This weapon was “Hedgehog,” a multispigot mortar 
which throws 24 projectiles fitted with contact fuzes 
so as to produce a pattern in the form of a ring whose 
center is about 250 yards ahead of the ship. The 
Hedgehog pattern, theoretically, has a greater chance 
of killing a submarine than the depth-charge pattern 
due to the fact that its design incorporates the fol- 
lowing three important principles: 

1. Ahead thrown. This permits a reduction in 
blind time from 45-90 seconds to 15-20 seconds. 

2. Multiple small charge. Three hundred pounds 
of high explosive (lethal range of 21 feet) has about 
one half the chance of killing the U-boat as ten 30-lb 
charges (lethal radius of 6 feet). The lower limit of 
the charge is the smallest weight which will have a 
reasonable chance of sinking the U-boat when it ex- 
plodes on the target. 

3. Contact firing. The contact fuze “sweeps” all 
depths. This is of particular importance in view of 
the fact that the antisubmarine gear used at that time 
did not determine the depth of the U-boat. 

However, against these advantages of Hedgehog 
must be considered the fact that the large explosion 
of depth charges has a desirable anti-morale effect 
and produces considerable shaking up of the U-boat 
in cases where the depth charges are not lethal. 
Hedgehog fails to do this since the charges explode 
only on hitting the U-boat. 

During this period, the results from radar were 
more promising as a consequence of increased train- 
ing and knowledge of the gear. A new set. Type 271, 
was being fitted on British corvettes. This was a short- 
wave (10 cm) “beam”-principle set and constituted a 
great advance in radar for antisubmarine purposes. 
The average range at which radar contact was made 
on a U-boat during this period was about 4000 yards, 
with the maximum range being 7000 yards. 

Coastal Command was also experimenting, during 
this period, with a searchlight, carried in a Welling- 
ton aircraft, to provide illumination for night attacks 
on U-boats. 

Sinkings of U-boats 

The steadily upward trend in the number of 
U-boats sunk or probably sunk monthly continued 
throughout this period, reaching a new high for the 


22 


START OF WOLF PACKS; END-TO-END OF CONVOYS 



Figure 2. Hedgehog ready to fire. 


war in December 1941 when 13 U-boats are believed 
to have been lost. However, it should be kept in mind 
that the rate of increase in the number of U-boats 
sunk was much lower than the rate of increase in the 
number of U-boats at sea, so that the average U-boat 
at sea had a much smaller probability of being sunk 
in this period than in either of the two preceding 
periods. 

The total number of enemy U-boats sunk during 
this 9-month period was 44. Thirty of these were lost 
in the Atlantic (22 German and 8 Italian) and 13 in 
the Mediterranean (6 German and 7 Italian). One 
Japanese U-boat was probably sunk in the Pacific 
during December. Two German U-boats were known 
to have been lost in the Baltic as a result of collisions 
with their own craft. 

Surface craft continued to be the most effective 
craft in sinking U-boats, accounting for 20 of the 30 
U-boats sunk in the Atlantic. Another two were sunk 


as a result of combined attacks by ships and aircraft. 
Two were lost as a residt of aircraft attacks and one 
was torpedoed by a submarine. The circumstances 
under which the other live U-boats, known to have 
been lost in the Atlantic, were sunk are not known. 

During the last six months of 1941, one out of 
every three U-boats attacked by surface craft was at 
least damaged, while one out of every seven attacked 
was sunk or probably sunk. 

3 3 SURVEY OF RESULTS 

3.3.1 From the U-boat’s Point of View 

The average number of U-boats at sea in the At- 
lantic during the third period was about 30, three 
times as many as in the previous period. Their main 
effort was directed in the form of wolf-pack attacks 
against the North Atlantic convoys. They did succeed 
in forcing the Allies to adopt complete end-to-end 


SURVEY OF RESULTS 


23 



Figure 3. DE firing Hedgehogs on shakedown cruise. 
Projectiles can be seen above the bow. 


escort of their transatlantic convoys but they defi- 
nitely failed in their main objective of cutting off 
supplies to England and toward the end of this 
period there were signs of their shifting to other 
areas. 

Despite this threefold increase in the average 
number at sea; the U-boats were able to sink only 
about 34 ships of about 166,000 gross tons monthly in 
the Atlantic during this period, or about 25 per cent 
less than in the previous period. This meant that the 
average U-boat was only sinking a little over one ship 
of about 5500 gross tons per month at sea, and was 
therefore only about one-fourth as effective as in the 
previous period. This drop in efficiency reflects the 
successful evasive routing of convoys and the very 
rapid expansion in U-boat personnel. This latter 
condition resulted in a high proportion of U-boats 
being sunk on their first cruise. The evasive routing 
of convoys also resulted in fewer contacts between 
convoy escorts and U-boats and consequently the 
average U-boat was relatively much safer during the 
third period. About 3% U-boats were sunk monthly 
in the Atlantic, of the 30 at sea on the average, and 
consequently the average life of a U-boat at sea was 
about 9 months, more than twice as long as it had 
been in the previous period. The average U-boat 
during this period was sinking ten ships of about 
50,000 gross tons before it was sunk itself. Hence, 
despite the longer lifetime of the average U-boat, the 
exchange rate was about 40 per cent less than in the 
previous period, reflecting the decreased effective- 
ness of U-boats in sinking ships. 


The number of ocean-going German U-boats in 
commission was expanding at a very rapid rate, in- 
creasing from about 54 at the beginning of this 
period to about 200 at the end of 1941. About 174 
German U-boats were commissioned during this 
period while only about 28 were lost. From the point 
of view of number alone, the U-boats offered a con- 
siderable threat and it seemed likely that with in- 
creased experiences the effectiveness of these new 
U-boats would increase. 


3-3.2 From the Allies’ Point of View 

Total shipping losses of the Allied and neutral 
nations decreased to 363,000 gross tons monthly dur- 
ing the third period, while the building rate of new 
shipping increased to about 175,000 gross tons 
monthly. Consequently, the net monthly loss of ship- 
ping was only 188,000 gross tons, about 45 per cent 
less than during the preceding period. 

Of the 363,000 gross tons of shipping lost monthly, 
about 323,000 gross tons were lost by enemy action. 
U-boats accounted for 175,000 gross tons a month, 
54 per cent of the total lost by enemy action. Monthly 
losses to enemy aircraft increased to 75,000 gross tons, 
while the losses to enemy surface craft dropped to 
only 17,000 gross tons. Mines were responsible for 
only 19,000 gross tons a month, while other and un- 
known causes accounted for 37,000 gross tons a 
month. 

The total shipping available decreased from about 
35,000,000 gross tons at the beginning of this period 
to about 33,300,000 at the end of 1941, but the up- 
ward trend in shipping losses seemed to have been 
checked. The number of British antisubmarine ships 
suitable for ocean escort had increased from about 
375 at the start of the period to about 500 at the end 
of 1941. In addition, the entry of the United States 
into the war added about 175 destroyers to the above 
numbers, but many of them were committed to 
action in the Pacific. 

From the defensive point of view the U-boats, 
which constituted the main threat to Allied shipping, 
seemed to have been defeated in the Battle of the 
Atlantic during this third period. They were experi- 
encing difficulty in locating Allied convoys and even 
when they did locate a convoy, the escorts were gen- 
erally able to beat off the wolf-pack attacks without 
serious losses. Shipping losses to U-boats had been 
kept to a reasonable level, considering the number of 


CQ NFTliins;^ ^' 



24 


START OF WOLF PACKS; END-TO-END OF CONVOYS 


U-boats at sea, while shipping construction was grad- 
ually increasing. However, from an offensive point of 
view, the U-boats were still relatively safe. Surface 
craft acting as convoy escorts were the only serious 
threat to the U-boat, although aircraft were gradu- 
ally becoming more effective in harassing and dam- 


aging U-boats. It seems, therefore, that the general 
situation in the Battle of the Atlantic at the end of 
1941 was roughly the same as the situation which 
prevailed at the end of World War I— that is, with 
shipping losses checked, but with the U-boats rela- 
tively safe at sea. 


CO 


Chapter 4 

FOURTH PERIOD 

HEAVY SINKINGS ON EAST COAST OF UNITED STATES 
JANUARY 1942-SEPTEMBER 1942 


^ ^ U-BOAT OFFENSIVE 

''T^HE GERMANS Started this period with about 200 
-L ocean-going U-boats and new U-boats were being 
commissioned at the rate of about 20 a month. Admi- 
ral Doenitz was therefore able to maintain a large- 
scale U-boat offensive over widely spread areas 
throughout this period. The average number of 
U-boats at sea in the Atlantic increased steadily from 
22 in January 1942 to 93 in September 1942. In addi- 
tion, there were about 20 U-boats in the Mediterra- 
nean and about 20 available for operations in the 
Barents Sea. These U-boats were used in a specific 
effort to cut supply lines to Allied forces in Libya and 
Russia. Japan, at the start of this period, had about 
75 U-boats which operated in the Pacific and Indian 
Oceans. 

Despite this widespread U-boat activity, the main 
battle continued to be fought in the Atlantic Ocean. 
There, during the previous period, U-boat opera- 
tions against escorted shipping had been steadily be- 
coming less and less profitable. The average yield 
had been reduced to about one ship sunk per U-boat 
month at sea during the last period. The operation 
against Convoy HG 76 in December 1941 had been 
particularly costly, as only two merchant ships were 
sunk as against four U-boats sunk. It was natural, 
therefore, after the entry of the U. S. into the war, 
that the U-boats, continuing their search for weak 
spots in the Allied defenses, headed westward for the 
American coast in January 1942. 

The U-boats, working their way down the Afneri- 
can coastline from the Newfoundland banks, found 
exactly the weak spot they were looking for. The de- 
mands of the war in the Pacific and commitments in 
transatlantic escort (including destroyers transferred 
to the British in 1940) contributed to the United 
States’ lack of preparedness for the scale of attack 
launched by the U-boats on the Atlantic coast in 
1942. The forces available to combat these enemy 
activities were relatively untrained and inexperi- 
enced. With the limited number of antisubmarine 


craft, both surface and air, at their disposal, the U. S. 
Navy was unable to start convoying coastal shipping 
immediately^ but tried during the early months of 
1942 to cover the long coastal route by patrol. This 
produced a number of attacks on U-boats but it 
failed to prevent extremely heavy losses of shipping 
sailing unescorted along the coast. 

U-boat activity in the West Atlantic began on 
January 12, 1942, when the first sinking west of 60° 
west longitude occurred. A force of about 20 U-boats 
began to operate off the Atlantic seaboard of the 
United States and in the coastal area of Nova Scotia 
and Newfoundland. These U-boats were rather se- 
lective in their choice of targets, preferring tankers 
and larger cargo ships and avoiding convoys. As long 
as worth-while targets abounded in the form of un- 
armed and unescorted ships, the U-boats kept clear of 
escorts, as even minor damage might well have pre- 
vented their return to distant bases. 

4 he U-boats inflicted their heaviest losses in Janu- 
ary in the Eastern Sea Frontier, sinking 14 ships of 
about 100,000 gross tons with a large proportion of 
the losses occurring at focal points of shipping such 
as Cape Hatteras, North Carolina, and Hampton 
Roads, Virginia. About 50,000 gross tons of shipping 
were sunk by U-boats in each of the Northwest Atlan- 
tic, Canadian Coastal, and Bermuda Areas. There 
was comparatively little activity in the remainder of 
the Atlantic, but Japanese U-boats sank 50,000 gross 
tons of shipping in the Pacific and Indian Oceans. 
The total losses for the month, 61 ships of 324,000 
gross tons sunk by U-boats, were higher than those in 
any month in the previous period. 

The situation became much worse in February 
1942 with the world-wide shipping losses to U-boats 
reaching a new high for the war as 82 ships of 470,000 
gross tons were sunk. About 90 per cent of these losses 
occurred in the U. S. Strategic Area as the number of 
U-boats operating in the West Atlantic increased and 
U-boat activity spread further south with ships being 
sunk off the coast of Florida and in the Caribbean 
Sea. Tanker losses continued to be severe, with the 


2 r, 




26 


HEAVY SINKINGS ON EAST COAST OF UNITED STATES 


tanker traffic to and from the West Indian and Vene- 
zuelan oil fields being an obvious objective of the 
U-boats. This was shown by an attack carried out by 
several U-boats on February 16 on six tankers off 
Aruba and in the Gulf of Venezuela, five being sunk 
and one seriously damaged. 

During March the U-boats continued the same 
tactics with increased success as they sank 94 ships of 
532,000 gross tons. The Eastern Sea Frontier con- 
tinued to be the most active area, with over 150,000 
gross tons of shipping sunk there by U-boats. Possibly 
by way of diversion, a group of U-boats operated in 
the Freetown Area, sinking over 50,000 gross tons 
there. 

The one encouraging feature of the month’s oper- 
ations was the first successful attacks on U-boats in 
the U. S. Strategic Area. Two U-boats were sunk in 
March as a result of attacks by U. S. Navy aircraft in 
the Canadian Coastal Zone. On April 15, USS Roper 
sank U-85 off Cape Hatteras, picking up 29 bodies, 
for the first confirmed sinking of a U-boat off the U. S. 
coast. The number of attacks on U-boats in the U. S. 
Strategic Area had increased from about 15 in Janu- 
ary to about 60 in April. 

These more effective countermeasures probably 
played some part in causing a small decrease in ship- 
ping losses in April, but a more important factor was 
the temporary suspension of sailings in certain areas. 
U-boat activity spread to the Brazilian Area during 
April as three ships were sunk off the north coast 
of Brazil, probably by Italian U-boats. 

In the middle of May 1942, the U. S. Navy was able 
to start convoying shipping along the east coast. 
The effect of the institution of these convoys was im- 
mediately apparent. The U-boats avoided escorted 
shipping and the tonnage sunk by U-boats in the 
Eastern Sea Frontier in May dropped to 23,000 gross 
tons. Logically enough, the U-boats sought out the 
remaining soft spots, where unescorted traffic had to 
pass through focal areas, and operated actively off the 
mouth of the Mississippi and in the Yucatan Channel 
between Cuba and Nicaragua. 

Although the average number of U-boats at sea in 
the Gulf Sea Frontier in May 1942 was only about 
four, these U-boats sank 41 ships of 220,000 gross tons 
there during the month, an all-time high for sinkings 
by U-boats in any area. The average number of ships 
at sea in the Gulf Sea Frontier was about 75, so the 
average life of a ship at sea at that time was less than 
two months at that rate of sinkings. 


Sinkings in the Caribbean Sea Frontier also in- 
creased, reaching about 170,000 gross tons in May. 
Consequently, despite the decrease in the Eastern 
Sea Frontier, the losses in the U. S. Strategic Area 
reached a peak of 1 16 ships of 567,000 gross tons sunk 
by U-boats in May. The world-wide shipping losses 
to U-boats also reached a new high for the war as 124 
ships of 604,000 gross tons were sunk during this 
month. The number of attacks on U-boats in the 
Western Atlantic showed a promising increase, how- 
ever, and two U-boats were sunk by U. S. Coast 
Guard cutters, one in the Eastern Sea Frontier and 
one in the Gulf Sea Frontier. 

The world-wide shipping losses to U-boats reached 
their highest point in the war in June 1942, when 141 
ships of 707,000 gross tons were sunk. The bulk of 
the increase over the figures for May was accounted 
for by increased activity by Japanese U-boats, which 
sank 70,000 gross tons in the Indian Ocean, mostly in 
the Mozambique Channel. The shipping losses in 
the U. S. Strategic Area were about the same as in 
May, falling off in the Gulf Sea Frontier but con- 
tinuing to increase in the Caribbean and Panama 
Sea Frontiers. Mines were laid in the Chesapeake, 
causing several casualties in June. This mine-laying 
may have represented the first effort of one of the 
1600-ton mine-laying U-boats introduced by Ger- 
many at about that time. Over 100 attacks were made 
on U-boats in the U. S. Strategic Area in June; three 
of these attacks resulting in sinking U-boats. 

During July 1942 the convoy system on the east 
coast of the U. S. was greatly extended, with the bulk 
of shipping traveling in convoy. This increase in the 
number of convoys and an improvement in the 
strength of escorts were probably mainly responsible 
for the general reduction in sinkings of shipping 
throughout the U. S. Strategic Area. Only 230,000 
gross tons of shipping were sunk by U-boats in the 
U. S. Strategic Area in July, less than half the amount 
sunk in June; this, despite the fact that the average 
number of U-boats at sea in the U. S. Strategic Area in 
July was about 45, higher than in any previous 
month. Increased aircraft patrols and better cooper- 
ation between surface and air units also contributed 
materially to this reduction of U-boat effectiveness in 
the West Atlantic. Another significant factor was the 
increase in the number of U. S. craft available for 
antisubmarine warfare; 134 ships in July as com- 
pared to 68 in April and 580 planes in July as com- 
pared to 350 planes in April. The increased effec- 


C 


U-BOAT OFFENSIVE 


27 


tiveness of U. S. countermeasures is also illustrated 
by the fact that the number of U-boats lost in the 
U. S. Strategic Area reached a new high in July as 
seven were sunk, two in Eastern Sea Frontier, one in 
Gulf Sea Frontier, two in Panama Sea Frontier, and 
two in the Northwest Atlantic Area. 

As a result of the greatly reduced losses in the West 
Atlantic, the world-wide shipping losses to U-boats 
dropped to 94 ships of 472,000 gross tons in July 1942. 
However, increased losses were suffered in the Free- 
town and Azores Areas with over 50,000 gross tons 
sunk by U-boats in each of these areas. Heavy losses 
were also suffered in July in the Barents Sea Area, 
where ten ships of 62,000 gross tons were sunk by 
U-boats from Convoy PQ 17 heading for Russia. In 
addition, 13 ships were sunk from this convoy by 
enemy aircraft. The convoy had been ordered to 
scatter when southeast of Spitzbergen to reduce losses 
from the enemy surface craft attack then apparently 
pending. 

The shipping losses to U-boats increased slightly 
in August to 108 ships of 544,000 gross tons, still well 
below the record highs established in May and June 
1942. This increase was due entirely to increased ac- 
tivity in the U. S: Strategic Area, where the losses 
mounted again to 84 ships of 407,000 gross tons sunk 
by U-boats. Over half of these losses occurred in the 
Caribbean Sea Frontier, where the main sore spots 
were the eastern approaches to Trinidad and the 
Windward Passage area. A group of about five 
U-boats achieved considerable success, especially 
against tankers, east of Trinidad. Twenty of the 23 
ships sunk there were sailing independently. In the 
Windward Passage area 14 ships were sunk from con- 
voys running between Key West and Trinidad and 
between Panama and Guantanamo. The activities of 
probably two U-boats off the Brazilian coast resulted 
in seven sinkings and brought Brazil into the war on 
the side of the United Nations. The coastal waters of 
the Atlantic from Nova Scotia to the tip of Florida 
were free from attacks by U-boats during the entire 
month of August. 

The other significant trend of the month’s opera- 
tions was the resumption of large-scale attacks on the 
transatlantic convoys. In view of the reduced effec- 
tiveness of U-boats in the Eastern and Gulf Sea Fron- 
tiers, due to the start of convoying and the heavy air- 
craft coverage, it was natural that the U-boats would 
resume their attacks on the transatlantic convoys, 
especially in areas in the North Atlantic out of the 


range of land-based Allied aircraft. Over 20 ships of 
about 120,000 gross tons were sunk by U-boats in the 
Northwest Atlantic Area in August. The heaviest 
losses were suffered by Convoy SC 94, which lost 1 1 
ships as a result of U-boat attacks. 

In these attacks on convoys, the first indication of 
the presence of U-boats was often an HF/DF bearing, 
and these were of great assistance to escort com- 
manders in appreciating the subsequent situation. 
Shipborne 10-cm radar (SG and Type 271) also 
proved a most efficient detector at night, and the 
quick action taken by escorts in many cases thwarted 
night attacks. This apparently influenced some of 
the U-boats to temporarily abandon their tactics of 
night surfaced attack and to attack submerged in 
daylight. As many as five ships were torpedoed by one 
salvo on a day attack, and to lessen the chance of such 
an event recurring instructions were issued in the 
third week of August to open out the distance be- 
tween the columns of a convoy to about 1000 yards 
by day as well as by night. 

The number of U-boats destroyed during August 
reached a new high for the war as 20 were sunk or 
probably sunk, 12 in the Atlantic, five in the Medi- 
terranean, and three in the Pacific. Two U-boats were 
sunk by escorts of Convoy SC 94 and a U-boat was 
sunk in the Caribbean Sea Frontier for the first time. 
U-464, sunk by a U. S. PBY plane southeast of Ice- 
land, turned out to be one of the new 1600-ton supply 
U-boats. These were used to refuel other U-boats at 
sea and thereby enabled them to extend their cruises. 
These supply U-boats were intended to stay at sea for 
as long as six months. 

At the end of August, the coastal convoy system 
was further extended and New York included as one 
of the ports in the system. In September, New York 
also became the main western port for the transat- 
lantic convoys, with HX and SC convoys beginning 
and ON convoys ending their passages there. An- 
other new development in September was the estab- 
lishment of a temporary reinforcing group whose 
primary objective was the destruction of U-boats 
rather than the immediate defense of shipping. This 
group consisted of ten British escorts. 

The shipping losses to U-boats decreased slightly 
in September to 99 ships of 496,000 gross tons. 
The principal areas of attacks on shipping were 
in the eastern approaches to Trinidad and in the 
area to the north and south of the Equator between 
Freetown and Ascension Island. There was also con- 




28 


HEAVY SINKINGS ON EAST COAST OF UNITED STATES 


siderable activity along the convoy routes in the 
Northwest Atlantic as well as in the Gulf of St. 
Lawrence. 

In the Northwest Atlantic Area, U-boats sank 20 
ships of 110,000 gross tons. The chief sufferer on the 
North Atlantic convoy routes was ON 127, with 
which five or six U-boats were in contact for four 
clays in succession. In the course of these attacks, two 
of which were made in daylight, 11 merchant ships 
and one escort were torpedoed, four of the ships man- 
aging to reach port. 

The two North Russian convoys had to fight their 
way through incessant U-boat and aircraft attacks, 
losing 16 ships in all, eight to U-boats. Six very prom- 
ising attacks were carried out by the escorts who were 
assisted by Swordfish aircraft from HMS Avenger. 
This was the first operation in which this type of 
escort carrier had taken part. The losses to U-boats in 
the Caribbean Sea Frontier in September 1942 were 
27 ships of 130,000 gross tons, considerably less than 
in August. A notable feature of the operations 
around the Trinidad Area was the sinking of ten 
ships engaged in the valuable bauxite trade. With 
the tremendous increase in shipping to and from the 
South Atlantic, the Trinidad Area had become one of 
the largest shipping focal points in the Western 
Hemisphere. U-boat activity had increased accord- 
ingly and was highly successful at first. Lack of suit- 
able long-range surface and air craft restricted the dis- 
tance that east-bound convoys could be escorted from 
Trinidad. Beyond the dispersal points the U-boats 
had good hunting. 

Aircraft available in the Trinidad Area, especially 
the RAF Squadron 53 (Hudsons), were utilized to 
the utmost and reinforcement aircraft from other 
areas, including a specially trained “Killer” group of 
U. S. Army Air Force B-18’s, were ordered into the 
area. Emphasis was placed on offensive operations, 
using sweeps over most likely U-boat locations and 
routes along which convoys passed. The results were 
rather encouraging in September as one U-boat was 
sunk and several other promising attacks were made. 
Fhe situation became somewhat eased after Septem- 
ber as the number of U-boats in the area started de- 
creasing. dlierefore, in just over nine months from 
their entry into World War II, the United States, 
by the institution of escorted convoys and the pro- 
vision of air cover and air patrols, had achieved a 
high degree of immunity from U-boat attack in their 
coastal waters. 


4 2 COUNTERMEASURES TO THE 

U-BOAT 

Convoys 

In January 1942, the U-boats transferred their at- 
tention from the transatlantic and East Atlantic con- 
voys to the unescorted shipping in the West Atlantic. 
This is reflected in the fact that only 10 per cent of 
the shipping sunk by U-boats during the first six 
months of 1942 was in convoy when sunk. This pro- 
portion increased again to about 30 per cent during 
the period July to September 1942 when the bulk of 
U. S. coastal shipping was being convoyed and the 
U-boats were attacking the transatlantic convoys 
again. 

It was realized during the early months of the war 
that convoying was the only solution to the heavy 
losses off the Atlantic Coast. However, the U. S. Navy, 
due to its commitments in transatlantic escort and in 
the Pacific, did not have enough escorts to start the 
convoying of coastal shipping at the beginning of 
1942. To provide additional forces, 24 British anti- 
submarine trawlers were allocated for service on the 
American coast and ten British corvettes were 
turned over to the U. S. Navy. Further, the whole 
system of transatlantic escort was recast and all anti- 
submarine forces (U. S. Navy, Royal Canadian Navy, 
and Royal Navy) were pooled in a single cross-Atlan- 
tic convoy scheme. This resulted in a certain econ- 
omy and released a limited number of U. S. de- 
stroyers. 

With the forces thus available and with the in- 
creased production of antisubmarine ships in the 
United States, it was possible to start convoying in 
the Western Atlantic in May 1942. Coastal convoys 
between Norfolk, Virginia, and Key West, Florida, 
started running on May 14. By opening up the trans- 
atlantic convoy cycle, the British were able to divert 
enough forces to the Caribbean to start convoys, 
mainly for tankers, over the Trinidad-Halifax and 
Aruba-Curacao-T r i n idad rou tes. 

During July, the convoy system on the east coast 
was greatly extended, with convoys running between 
Trinidad and Key West by way of Curacao and 
Aruba. Convoys were instituted between Panama 
and Guantanamo to connect with the other convoys. 
Convoys were also started in the Gulf of Mexico and 
the Gulf of St. Lawrence. 

At the end of August, convoys running in both 
directions between Curacao and Halifax and be- 


COUNTERMEASURES TO THE U-BOAT 


29 


tween Key AVest and Trinidad were discontinued, as 
were also the convoys between Hampton Roads and 
Key West. A new convoy system was started with 
convoys running between New York and Guanta- 
namo [NG and GN], and between the latter port and 
Trinidad by way of Curacao [GAT and TAG]. Con- 
voys were also started between New York and Key 
West [NK and KN]. 

During the five months from May through Sep- 
tember 1942 about 1800 ships were convoyed 
monthly in the U. S. coastal convoys and only about 
12 of these ships were sunk monthly by U-boats for 
a loss rate of less than 1 per cent per trip. During the 
first nine months of 1942, about 1000 ships were con- 
voyed monthly in ocean convoys and about 15 of 
these ships were sunk monthly by U-boats for a loss 
rate of about li/^ per cent per trip. (It should be con- 
sidered that the average voyage for these convoys is 
considerably longer than for the coastal convoys.) 
The convoy run to and from Russia was particularly 
hazardous during this period, with about 34 ships 
sailing monthly and about three of these being sunk 
monthly by U-boats for a loss rate of about 9 per cent 
per trip. In addition, these convoys suffered consider- 
able losses from enemy air and surface craft attack. 

The effect of convoying in reducing shipping losses 
is clearly illustrated by the experience in the U. S. 
Strategic Area during the first nine months of 1942. 
There were about 600 ships at sea in this area 
throughout this period. During the first six months, 
before extensive convoying of coastal shipping had 
started, only about 40 per cent of the shipping was in 
convoy. There were, on the average, about 30 U-boats 
at sea in this area during the first six months and each 
U-boat was sinking about 2.7 ships a month. About 
20 per cent of the independent shipping and about 
4 per cent of the convoyed shipping were sunk each 
month by U-boats. 

During the next three months, after extensive con- 
voying of coastal shipping had started, about 80 per 
cent of the shipping was in convoy. The average 
number of U-boats at sea in this area had increased 
to about 50, but each U-boat was only able to sink 
about 1.4 ships a month, about half as much as dur- 
ing the first six months. Thus, despite the fact that 
the loss rates for both independent and convoyed 
shipping had increased (about 33 per cent of the in- 
dependent shipping and about 6 per cent of the con- 
voyed shipping were sunk monthly by U-boats) dur- 
ing the last three months of this period, the efficiency 


of the average U-boat in sinking ships was halved. 
This was mainly due to the fact that about 40 per 
cent of the shipping was exposed, during the latter 
three months, to the much lower loss rate experi- 
enced by convoyed shipping instead of to the high 
loss rate experienced by independent shipping. 

Another consequence of the shift from independ- 
ent to convoyed shipping is the increased danger 
which the U-boat faces when he attacks a convoy. 
Only about ly^ U-boats were sunk monthly in the 
U. S. Strategic Area during the first six months of 
1942, while 4i^ were sunk monthly in the same area 
during the next three months, most of them by forces 
escorting convoys. 

Aircraft 

At the beginning of 1942, the U. S. Navy sent out 
all available planes and blimps to battle the U-boats 
along the coast. They were helped by the First 
Bomber Command, initial Army Air Forces contri- 
bution, which was activated in December 1941. The 
Army planes patrolled and escorted under the opera- 
tional control of the Navy. A second Army Air Forces 
unit broke into the picture in June, 1942. This was 
the Seasearch-Attack Development Unit [SADU], 
based at Langley Field, Virginia, and assigned a 
combination mission: (1) to develop tactics and tech- 
niques for using antisubmarine devices, and (2) to 
conduct general seasearch. SADU had two British 
loaned B-24's (Dumbos I and II) . In these had been 
installed two early British microwave sets known as 
DMS-1000 and equipped with the first airborne 
Plan Position Indicator [PPI] scopes. In addition to 
the Army and Navy flying, there was also patrolling 
by the Civilian Air Patrol [CAP], mostly within 100 
miles from shore. 

The flying hours by U. S. Army and Navy aircraft 
in the Eastern Sea Frontier increased from about 
5000 hours in January 1942 to a peak of about 25,000 
hours in July 1942. In the Gulf Sea Frontier, only 
about 7000 hours were flown in May 1942, when sink- 
ings of ships were at their peak, as compared to 12,000 
hours in July 1942. In both of these sea frontiers, the 
number of U-boats at sea decreased rapidly after 
July 1942. In the Caribbean Sea Frontier, the num- 
ber of flying hours increased from about 5000 in 
April 1942 to almost 10,000 in September 1942, when 
the number of U-boats at sea there started decreasing. 

U. S. aircraft made about 30 attacks a month on 
U-boats during this period, varying from about 12 a 


30 


HEAVY SINKINGS ON EAST COAST OF UNITED STATES 


month during the first four months of 1942 to about 
45 a month during the next five months. About 20 
per cent of these attacks resulted in some damage to 
the U-boat, while only about 2 per cent of the attacks 
resulted in the sinking or probable sinking of the 
U-boat. The average height at which these attacks 
were made was about 150 feet. The depth bombs 
used were generally set for a depth of 50 feet at the 
beginning of this period (too deep for a surfaced 
U-boat) but toward the end of 1942 most of the at- 
tacks were made with settings of 25 feet. In addition, 
bombs were fitted with flat noses in order to reduce 
their forward motion under water and also to reduce 
ricocheting. 

At about the beginning of 1942, Coastal Command 
aircraft had started using the 25-foot depth setting on 
their depth charges. The usual height at which their 
attacks were made was about 50 feet. Torpex-filled 
depth charges, which had a greater lethal radius, 
were introduced in April 1942. These factors pro- 
duced a considerable improvement in the quality of 
Coastal Command attacks on U-boats. About 20 per 
cent of their aircraft attacks during this period re- 
sulted in at least some damage to the U-boat, while 
about 4 per cent of their attacks resulted in sinking 
the U-boat. 

During the early months of 1942, when most of the 
U-boats were operating in the West Atlantic, Coastal 
Command aircraft started maintaining offensive 
patrols against transit U-boats. They operated both 
on the northern route, to the northward of Scotland, 
and in the approaches to the Bay of Biscay ports. 
During the first five months of 1942 only about 600 
flying hours a month were spent on the Bay of Biscay 
offensive. This effort resulted in about seven sight- 
ings a month, enough to keep the U-boats submerged 
during the daytime. 

In June 1942 Coastal Command introduced into 
operation about ten Wellington aircraft, fitted with 
the Leigh Searchlight and Mark II radar. These air- 
craft, operating in the Bay of Biscay, flew 190 hours 
in June, sighted seven U-boats, and attacked five of 
them. This success was achieved despite the fact that 
the night offensive in the Bay was considerably ham- 
pered by the presence of French fishing boats. 

With the introduction of the Leigh-Light Welling- 
ton in June 1942 the scale of the Bay offensive was 
greatly increased, with about 3400 flying hours being 
put into it monthly during the period from June 
through September 1942. About 30 sightings (9 per 


1000 hours on patrol) and 23 attacks were made 
monthly during this period. It is believed that 43 per 
cent of all U-boat transits through the Bay were 
sighted and that five U-boats were destroyed during 
those four months. The immediate reaction of the 
enemy to the Coastal Command offensive in the Bay 
was an increased effort to intercept antisubmarine 
aircraft, to which Coastal Command replied by send- 
ing out Beaufighters to intercept the interceptors. 

^ Scientific and Technical 

The Germans were fully alive to the possibilities 
of meter-wave radar and they were aided by the cap- 
ture of a Mark II radar set in Tunisia in the spring of 
1942. They accordingly concluded that radar was re- 
sponsible for the night attacks in the Bay of Biscay 
and tests in the summer of 1942 confirmed that the 
transmissions were easily detected by a single receiver 
and aerial. Admiral Doenitz ordered the speediest 
equipping of all U-boats with a makeshift equip- 
ment. The aerials of wood and cable (Southern Cross 
Aerial) were easily made and the Paris firm of Metox 
turned out the R-600 receiver. The first U-boat Ger- 
man Search Receiver [GSR] was designed to detect 
meter radar and came into operation about October 
1942. 

Another new device introduced by the U-boats in 
the latter half of 1942 was Submarine Bubble Target 
[SBT] or “Pillenwerfer.” These were tablets which 
were to be released by the U-boats when attacked by 
surface craft. The bubbles formed by the dissolving 
tablets produced false sonar targets which were in- 
tended to throw the attacking ships off the trail of 
the U-boat. 

In the early months of 1942, a new form of passive 
defense against torpedo attacks, known as Admiralty 
Net Defense [AND], was being fitted to new mer- 
chant ships of speeds not less than 1 1 knots and not 
more than 15 knots. Early trials indicated that these 
nets would stop about 50 per cent of U-boat torpedoes 
fired at the ship. 

In March 1942, the Hedgehog charges were made 
more lethal by filling them with Torpex, a new ex- 
plosive which was, volume for volume, 1.7 times as 
powerful as TNT. About the middle of 1942, the 
U. S. Navy put into operation the “Mousetrap” pro- 
jector, which fires a number of relatively small, fast- 
sinking charges equipped with contact fuzes and is 
suitable for installation on small antisubmarine 


SURVEY OF RESULTS 


31 


ships. As the light construction of these ships does 
not permit any great amount of deck thrust, the pro- 
jector utilizes the rocket principle for launching. The 
projectiles are copies of the Hedgehog projectiles 
fitted with suitable rocket motors in the tail. 

Magnetic Airborne Detector [MAD] was also de- 
veloped during this period in order to enable air- 
craft to follow submerged submarines. This device 
detects the change in magnetic field produced by a 
submarine but its detection range is only about 500 
feet. 


Sinking of U-boats 

The number of enemy U-boats sunk during this 
9-month period was 78 (50 German, 17 Italian, and 
11 Japanese). Forty of the U-boats were lost in the 
Atlantic (21 of them in the U. S. Strategic Area), 21 
in the Mediterranean, 1 1 in the Pacific, two in the 
Barents Sea, one was mined in the Baltic, and three 
were lost under unknown circumstances. 

Surface craft continued to be the main factor in 
sinking U-boats in all areas, accounting for 34 
U-boats (44 per cent of the total number sunk) while 
coordinated attacks involving both surface and air- 
craft accounted for another six U-boats (8 per cent). 
Aircraft played a significant part in sinking U-boats, 
for the first time, accounting for 19 (24 per cent); all 
of these sinkings occurred in the Atlantic and Medi- 
terranean. Submarines were an important factor in 
both the Mediterranean and Pacific, accounting for 
14 U-boats (18 per cent of the total). 

In the U. S. Strategic Area, the main scene of 
U-boat activity, there were about 360 surface craft 
attacks made on U-boats during this period. About 
12 per cent of these attacks resulted in at least some 
damage to the U-boat while about 4 per cent of them 
resulted in sinking it. A British study of 106 surface 
craft attacks in the North Atlantic and Western 
Mediterranean during this period indicates that 
about 25 per cent of the attacks resulted in at least 
some damage to the U-boat while about 10 per cent 
of these resulted in sinking it. 

SURVEY OF RESULTS 

4.3.1 From the U-boat’s Point of View 

This was by far the most successful period of the 
war for the U-boats. The total world-wide losses 


amounted to 878 ships of 4,587,000 gross tons sunk 
by U-boats (about 98 ships of 510,000 gross tons sunk 
monthly). The main U-boat battle continued to be 
fought in the Atlantic where about 90 per cent of 
these losses occurred. The number of U-boats lost 
during this period was 78 (about nine a month) so 
that the world-wide exchange rate was 11 ships of 
59,000 gross tons sunk for each U-boat sunk. 

Japanese U-boats sank 76 ships during this nine- 
month period (48 in the Indian Ocean and 28 in the 
Pacific) while 1 1 U-boats were sunk (all in the Pacific) 
so that their exchange rate was about seven ships 
sunk for each U-boat sunk. In the Barents Sea Area, 
15 ships were sunk by U-boats as against two U-boats 
sunk while in the Mediterranean, 17 ships were sunk 
as against 21 U-boats lost. Life was much more haz- 
ardous for U-boats in the Mediterranean than for 
those in any other area. 

In the Atlantic, the main theater of activity, the 
average number of U-boats at sea was about 57, 
almost twice as many as during the previous period. 
These U-boats sank about 85 ships of 456,000 gross 
tons a month during this period, more than 2i/^ 
times as much as in the previous period. About 85 
per cent of these losses occurred in the U. S. Strategic 
Area, where their main effort was directed against 
the weak spot off the east coast of the United States. 
The average U-boat in the Atlantic sank about I 1/2 
ships of 8000 gross tons per month at sea, about 50 
per cent more than the corresponding figure for the 
previous period. However, it should be kept in mind 
that this sinking rate was still far below that achieved 
during the period from July 1940 to March 1941, 
when the U-boat Aces were operating and the average 
U-boat was sinking four ships per month at sea. 

The U-boats operating in the Atlantic were rela- 
tively safer during this period than at any other time 
in the war. Of the 57 U-boats at sea, about 4i/^ were 
sunk monthly so that the average life of a U-boat at 
sea reached a new high of 13 months. This meant 
that the average U-boat in the Atlantic, during this 
period, was sinking 19 ships of about 100,000 gross 
tons before it, itself, was sunk, the highest exchange 
rate of the war (about twice as high as during the 
previous period). 

This record exchange rate was achieved by con- 
centrating the U-boat effort in the U. S. Strategic 
Area, of the United States, where the defenses were 
rather weak and the bulk of shipping unescorted. Of 
the 57 U-boats at sea in the Atlantic, about 37 were 




32 


HEAVY SINKINGS ON EAST COAST OF UNITED STATES 


in the U. S. Strategic Area, where they were able to 
sink 71 ships of 375,000 gross tons a month while 
only 2% U-boats were lost monthly. 

During the first half of 1942, the U-boats achieved 
their main successes in the Eastern and Gulf Sea 
Frontiers. The number of U-boats in these areas 
reached a peak in July 1942 when there were, on the 
average, eight at sea in the Eastern Sea Frontier and 
six in the Gulf Sea Frontier. However, by then the 
bulk of shipping was in convoy and surface and air- 
craft had each been making over two attacks a month 
on each U-boat. Under these conditions, the effective- 
ness of U-boats in sinking ships was greatly reduced, 
and the number of U-boats operating in these coastal 
regions decreased rapidly after July 1942. In the 
Caribbean, it was not until August 1942 that a cor- 
responding level of attacks on U-boats was reached 
and the reduced effectiveness of the U-boats in sink- 
ing ships first became apparent in September 1942, 
when there was a peak of about 10 U-boats at sea 
there. The number of U-boats at sea in the Carib- 
bean started falling off after September 1942. 

It was natural, therefore, that as the combination 
of convoying and heavy air coverage reduced the 
effectiveness of U-boats along the East Coast of the 
United States, the U-boats would look for other weak 
spots in the Allied defenses. The most likely looking 
spot was the “gap” in the Northwest Atlantic Area 
where the U-boats could operate against the vital 
transatlantic convoys in a region outside the range of 
Allied air cover. The number of U-boats in this area 
had increased from about seven during the first half 
of 1942 to 14 in August and September while the 
shipping losses to U-boats in the Northwest Atlantic 
Area had mounted to over 100,000 gross tons in each 
of these months. It seemed likely that the crucial 
battle of the U-boat war would be fought against 
these convoys in the North Atlantic during the next 
period. 

The Germans were in an excellent position to con- 
duct an intensive U-boat campaign at the end of 
September 1942. The number of ocean-going U-boats 
available had increased from about 200 at the begin- 
ning of 1942 to about 350 at the end of this period. 
This was accomplished by commissioning about 200 
new U-boats while only 50 were lost. In addition, 
many of the new U-boat commanders and crews had 
gained considerable experience and confidence as a 
result of their successful operations in the West 
Atlantic. 


4.3.2 From the Allies’ Point of View 

Total shipping losses from all causes of the Allied 
and neutral nations reached the highest level of the 
war during this period, amounting to 700,000 gross 
tons a month, almost twice as high as during the 
previous period. Fortunately, the building rate of 
new shipping was also greatly increased, averaging 
about 515,000 gross tons a month. This increase re- 
flected chiefly the great expansion in U. S. construc- 
tion of shipping from less than 100,000 gross tons in 
January 1942 to almost 700,000 gross tons in Sep- 
tember 1942. Consequently, the net monthly loss of 
shipping was only about 185,000 gross tons, slightly 
less than during the preceding period. 

Of the 700,000 gross tons of shipping lost monthly, 
about 655,000 gross tons were lost as a result of enemy 
action. U-boats accounted for 510,000 gross tons a 
month (about 78 per cent of the total lost by enemy 
action), a much higher proportion than in the past. 
Monthly losses to enemy aircraft dropped to 67,000 
gross tons (10 per cent of the total). Monthly losses to 
enemy surface craft were about 39,000 gross tons, 
while the monthly losses to mines were only about 

11.000 gross tons. 

The total shipping available decreased from about 

33.300.000 gross tons at the beginning of 1942 to 
about 31,600,000 gross tons at the end of this period. 
Particularly serious was the heavy destruction of 
tankers during this period, 190,000 gross tons lost 
monthly as compared to 70,000 gross tons constructed 
monthly. The size of the tanker fleet declined by over 
1,000,000 gross tons, or about 10 per cent of its size at 
the beginning of 1942. 

The number of ships suitable for ocean escort, 
available to the Allies, increased from about 670 at 
the beginning of 1942 to 745 at the end of September. 
This was due mainly to an increase of about 60 in 
the number of destroyers. Twelve new auxiliary air- 
craft carriers were also completed during this period. 
However, the gravity of the U-boat situation at the 
end of this period resulted in a sizeable expansion in 
the construction program for 1943, with the U. S. 
Navy expecting to produce over 500 escort ships, 
more than half of which would be destroyer escorts 
[DE], designed especially for convoy escort and anti- 
submarine warfare. 

Although the Allies had suffered extremely heavy 
losses in the Western Atlantic during this period, 
convoying and aircraft had succeeded in driving the 


SURVEY OF RESULTS 


33 


U-boats out of these coastal regions by the end of this 
period. The main problem facing the Allies during 
the next period was that of maintaining the flow of 
war material from the United States to England, in 
particular, that of assuring the safety of the North 


Atlantic convoys. It was becoming increasingly ap- 
parent that, although the defeat of the U-boats 
would not, of itself, win the war, the Allies could 
not possibly win the war without first defeating the 
U-boats. 


Chapter 5 

FIFTH PERIOD 


LARGE WOLF PACKS BATTLE NORTH ATLANTIC CONVOYS 

OCTOBER 1942-JUNE 1943 

5.1 U-BOAT OFFENSIVE carried out six attacks; the attack on the convoy did 

not develop. On the other hand. Convoy SC 104 was 


D espite the fact that the U-boats had been driven 
from the coastal areas of the United States by the 
beginning of this period, the shipping losses to 
U-boats continued to run at the same high level as 
during the previous period. The Atlantic continued 
to be the chief area of U-boat activity with about 85 
per cent of the shipping losses to U-boats occurring 
there. There were over 100 U-boats at sea in the At- 
lantic during most of this period. This enabled the 
enemy to conduct an intensive U-boat campaign in 
the Northwest Atlantic Area against the strategically 
important North Atlantic convoys and still maintain 
a number of subsidiary campaigns in other wide- 
spread areas (such as the Caribbean, Brazilian, Free- 
town, and Southeast Atlantic Areas) in an attempt to 
force the Allies to disperse their forces. 

The total shipping losses to U-boats continued to 
be rather heavy in October 1942 as 93 ships of 614,000 
gross tons were sunk. The intensity of the U-boat 
campaign against the transatlantic convoys increased 
with the number of U-boats in the Northwest Atlan- 
tic Area rising from 14 in September to 22 in October. 
These U-boats sank 24 ships with the bulk of the 
losses occurring in the “gap,” the region outside the 
range of shore-based aircraft. Experience indicated 
that a convoy, not protected by aircraft, may be so 
disorganized by a concentrated attack and the re- 
sultant breaks in formation for rescue work and 
other adjustment that the escorts may become com- 
paratively ineffective for either protection or offense. 
However, the presence, even for a few hours, of one 
or two aircraft has again and again prevented a con- 
centrated attack from developing. 

This was well illustrated during October. On the 
4th, Convoy HX 209 was in some peril when about 
300 miles south of Iceland. U. S. Navy Catalinas pro- 
vided close escort for 15i/2 hours and protective 
sweeps w^re laid on by seven aircraft from Iceland 
and three Fortresses from England. In the course of 
these operations the aircraft made nine sightings and 


heavily attacked on the nights of the 13th and 14th 
when out of the range of aircraft and eight ships were 
torpedoed. On the following day there was a change 
for the better. The weather improved and air cover 
arrived enabling the surface escort to go over to the 
offensive and sink two U-boats. 

Another development during October was the 
start of the expected U-boat operations in the South- 
east Atlantic Area, near Capetown. This new “soft 
spot” was exploited by two groups, each of about six 
U-boats, which reached the Capetown area and even- 
tually passed into the southern part of the Indian 
Ocean, where Japanese U-boats had previously 
achieved considerable success. These groups sank 25 
ships in the Southeast Atlantic Area during October. 
All of these ships were independently routed and 
some carried rather valuable cargoes. 

However, some of the heavy shipping losses suf- 
fered in October and November may be charged to 
the success of Operation Torch, the landings in 
North Africa early in November 1942. A great deal of 
merchant shipping and many escorts were naturally 
diverted from their normal use in October, when the 
foundations were firmly laid for the achievement of 
the safe arrival of the first military convoys. In addi- 
tion, the enemy disposed an abnormally large pro- 
portion of his Atlantic U-boat force east of the 
Azores and Madeira. This suggests that, in a broad 
sense, the German command had appreciated the 
likelihood of an Allied attack on Africa. One conse- 
quence directly resulting from this redisposition of 
U-boat forces was that Convoy SL 125 was severely 
mauled, losing 12 ships during a 4-day pursuit by a 
pack of about six U-boats. However, the pursuit was 
abruptly dropped before it drew the U-boats away 
from the area southwest of Portugal; it may have 
been regarded as of greater importance that they 
should stay there and reconnoiter than that they 
should further pursue an ordinary trade convoy. 

Despite the fact that convoys of the expeditionary 


34 


U-BOAT OFFENSIVE 


35 


forces had to pass through concentrations of 30 to 
40 U-boats before reaching Gibraltar, no U-boat suc- 
cesses were achieved against these convoys until after 
the assault troops had landed. The number of 
U-boats in the Western Mediterranean increased 
from about 10 on November 8 to about 20 on No- 
vember 1 1. The losses to U-boats in Operation Torch 
amounted to only about 84,000 gross tons of shipping 
and six naval vessels. The total shipping losses from 
all enemy causes in this operation amounted to about 
134,000 gross tons, against which can be offset a gain 
to the Allies of about 181,000 gross tons of serviceable 
tonnage acquired in French ports. In addition to the 
defensive success achieved against the U-boats in 
Operation Torch, the Allies also scored a notable 
offensive success, sinking 15 U-boats in the Mediter- 
ranean during November. A large factor in these vic- 
tories over the U-boats was the heavy air cover pro- 
vided, since aircraft from Gibraltar made 110 sight- 
ings and 64 attacks on the large concentration of 
U-boats. 

In contrast to the successful landings in North 
Africa, world-wide shipping losses reached their 
highest point of the war in November 1942, as 862,- 
000 gross tons of shipping were lost from all causes. 
U-boats also sank the greatest tonnage of the war in 
November, accounting for 116 ships of 712,000 gross 
tons. The tonnage sunk during October and Novem- 
ber was swelled by the loss of a number of large, fast, 
independently routed ships which, on account of 
their speed, had been regarded as fairly safe from 
U-boat attack. Losses in the Northwest Atlantic con- 
tinued at their high level as 26 ships of 144,000 -gross- 
tons were sunk there by U-boats. Losses in the South- 
east Atlantic fell off slightly to 23 ships of 127,000 
gross tons but shipping losses in the Caribbean and 
Brazilian Areas mounted again as the U-boats sank 
34 ships of 210,000 gross tons there. These heavy 
losses in widespread areas occurred despite the in- 
tensive effort made by the U-boats in opposing the 
Allied landings in North Africa. 

The shipping losses to U-boats in December 1942 
were 62 ships of 344,000 gross tons, only about half 
the record total of November. This steep decline was 
probably due to the cumulative effect of various con- 
tributing factors, such as the concentration of 
U-boats in the Atlantic approaches to the Western 
Mediterranean and a consequent withdrawal else- 
where (particularly from the West and South Atlan- 
tic), the heavy bombing by Allied aircraft of the 


Biscayan U-boat bases, and some effective work by 
surface and air escorts on the North Atlantic routes, 
as well as adverse weather and some luck in routing. 
The enemy attributed the smaller sinkings to bad 
weather. 

Only one convoy, ONS 154, suffered heavy losses in 
the North Atlantic. This convoy was given a south- 
erly route and was attacked by a pack of about 20 
U-boats as it drew out of the range of air cover. These 
U-boats followed the convoy for four days and sank 
14 ships. This again demonstrated the immense im- 
portance of efficiently combining surface and air 
escort and indicated that surface escorts are physi- 
cally incapable of warding off concerted attacks 
by a pack of U-boats which outnumbers them by 
more than a 2-to-l ratio. In selecting convoy routes 
at this time, the advantage of the northerly route 
within the range of air cover from Iceland had to be 
balanced against the increased time of the trip due to 
greater distance and worse weather. In addition the 
bad weather reduced the efficiency of the escorts. 

The shipping losses to U-boats in January 1943 
dropped to only 35 ships of 203,000 gross tons, less 
than in any month of 1942. Bad weather was the pri- 
mary factor responsible for this reduction and it was 
recognized that the gravity of the U-boat threat was 
much more serious than indicated by the low ship- 
ping losses. The fate of a tanker convoy bound from 
Trinidad to Gibraltar seemed a truer index of the 
strength of a U-boat pack operating in favorable con- 
ditions at the beginning of 1943. Convoy TM 1, made 
up of nine tankers and four escorts, lost seven tankers 
as a result of U-boat attacks, all well outside the 
range of shore-based aircraft. This was the highest 
proportion of ships sunk from any convoy by U-boats 
during the war. 

Of the 100 or more U-boats at sea in the Atlantic 
on any one day, over a third were operating in the 
Northwest Atlantic Area. The U-boats were concen- 
trating against the North Atlantic convoys which 
carried cargoes of immediate importance for mili- 
tary operations. Grand Admiral Doenitz had become 
supreme commander of the German Navy and it was 
becoming evident that the critical phase of the 
U-boat war in the Atlantic was approaching. 

To meet to some extent the need for shore-based 
air cover for Atlantic convoys, the Allies planned to 
introduce Very Long Range [VLR] aircraft with an 
endurance of 2000 to 2500 miles. Some flying was also 
done from Greenland although weather conditions 


36 


LARGE WOLF PACKS BATTLE NORTH ATLANTIC CONVOYS 


were a severe handicap. Continuous shore-based air 
cover was available between Gibraltar and Freetown. 

In February 1943 the U-boat effort in the North 
Atlantic increased in intensity. The world wide ship- 
ping losses to U-boats increased to 63 ships of 359,000 
gross tons, with about half these losses occurring in 
the Northwest Atlantic Area. Two of the North At- 
lantic convoys were heavily attacked by packs of 
about 20 U-boats. Convoy SC 118 lost 12 ships to 
U-boat attacks early in the month, seven of them 
during a period of three hours. Flowever, at least 
two U-boats were sunk and six damaged during this 
action. Convoy ON 166 suffered the loss of 14 ships 
over a period of five days as a result of U-boat attacks 
outside the range of shore-based aircraft. The enemy 
was concentrating his forces in an effort to interrupt 
the flow of supplies from America to Great Britain 
and the U-boats were displaying increased boldness 
in attempting to achieve this objective. However, 
about 20 VLR aircraft had become operational in 
February 1943. 

In March 1943 the world-wide shipping losses to 
U-boats mounted to 107 ships of 627,000 gross tons. 
The tempo of the U-boat offensive against the trans- 
atlantic convoys increased as the number of U-boats 
in the Northwest Atlantic Area rose to about 50. 
These U-boats sank 38 ships in that area, a high for 
the war, and an additional 15 ships were sunk in the 
adjacent Northeast Atlantic Area. The enemy’s re- 
sources were such that he was also able to maintain 
offensives in the South Atlantic and in the Carib- 
bean and Azores Areas. About two-thirds of the ton- 
nage sunk by U-boats during March was in convoy 
when sunk and the figure of over 400,000 gross tons 
of shipping sunk in convoy by U-boats during the 
month was a record for the war. The main reason for 
the large increase in tonnage sunk in convoy was an 
increase in the number of convoys attacked and not 
a weakening in the protective value of the escorts. 

The intensity of the U-boat effort in the North 
Atlantic during March is clearly indicated by the at- 
tacks on four consecutive eastbound transatlantic 
convoys. The U-boats generally outnumbered the es- 
corts by about two to one and were able to torpedo 
30 ships from these four convoys, in addition to eight 
stragglers. Convoy SC 121 was badly scattered by 
heavy weather and lost five ships in convoy and seven 
stragglers. Convoy HX 228 lost four ships to the 
U-boats but the escorts were able to sink two U-boats 
in one of the dramatic incidents of the U-boat war. 


After an attack on the convoy, HMS Harvester 
sighted a U-boat and rammed it at about 26 knots. 
The U-boat hung under the stern and could not be 
dislodged for about ten minutes. The FFS Aconit 
finished off the damaged U-boat. In spite of con- 
siderable damage, HMS Harvester was able to pro- 
ceed on one engine for several hours until she became 
completely disabled and was torpedoed. The FFS 
Aconit then sank the U-boat which had destroyed 
HMS Harvester. 

Convoys HX 229 and SC 122 were routed closely 
together and ran into a concentration of about 40 
U-boats, the largest pack employed up to that time, 
losing 20 ships to the U-boats. On the night of March 
16-17, 13 ships were sunk when the convoys were 
about 850 miles from the nearest air bases. The fol- 
lowing day the convoys received some air cover from 
VLR aircraft, with one plane sighting six U-hoats. 
In all, 54 sorties were flown in defense of these two 
convoys resulting in 32 sightings and 21 attacks. The 
U-boats were finally forced to disengage on March 
20 and this date may well be considered as the turn- 
ing point in their offensive power as sinkings of mer- 
chant vessels were at a much lower rate thereafter. 

Although the shipping losses in March were very 
heavy, there were signs that the situation in the 
North Atlantic was improving. Advances had been 
made towards the provision of continuous air cover 
for transatlantic convoys by the increased use of VLR 
aircraft from Newfoundland, Iceland, and England 
and by the introduction of escort carriers and Mer- 
chant Aircraft Carriers [MAC] ships. MAC ships 
were merchant ships whose decks were converted so 
that they could carry four Swordfish aircraft which 
could take off and land. The USS Bogue, the first 
U. S. escort carrier, started operations as an escort 
for the North Atlantic convoys during March. At 
the Atlantic Convoy Conference held in Washington 
in March, it was agreed that, starting in May, the 
United Kingdom and Canada should be responsible 
for the security of convoys across the North Atlantic, 
the United States providing certain air and sea forces 
to help them. It was also decided to set up a unified 
air command in Newfoundland corresponding to the 
Coastal Command system. The United States was to 
base a substantial force of VLR aircraft in this re- 
gion, which, together with the VLR Liberators of the 
RCAF, were to close the gap in the North Atlantic. 

yVlthough the number of U-boats at sea continued 
at the same high level in April 1943, the shipping loss 


U BOAT OFFENSIVE 


37 


was greatly reduced with the U-boats sinking 56 ships 
of 328,000 gross tons. The shipping losses to U-boats 
in the Northwest Atlantic Area in April were only 
18 ships of 108,000 gross tons, less than half the 
March record. Ten ships were sunk by U-boats in the 
Freetown Area and Japanese U-boats sank six ships 
in the Southwest Pacific, off the east coast of 
Australia. 

Accompanying the decrease in shipping losses 
there was a considerable increase in the number of 
U-boats sunk, 17 being sunk in the Atlantic for a 
record high to that date. April also marked the first 
sinking of a U-boat in which planes from an escort 
carrier took part. A plane from HMS Biter found 
and attacked a U-boat which HMS Pathfinder, one 
of the escorts, sank. 

A more satisfactory criterion that the U-boat offen- 
sive strength had passed its peak was the fact that, 
for the first time, U-boats failed to press home attacks 
on convoys when favorably situated to do so. In none 
of the attacks on transatlantic convoys did the enemy 
succeed in obtaining anything like the upper hand. 
All of the U-boats’ efforts tended to avoidance of de- 
tection and, once it was apparent that they had failed 
in this endeavor, they seldom pressed home their 
attacks. There were five Support Groups operating 
in the North Atlantic during April, two of them 
having their own escort carrier, and the number of 
VLR aircraft available had risen to over 30. More 
important, there was a noticeably enhanced standard 
of group training, better use of HF/DF was made, 
and cooperating between surface and air components 
of the escorts was greatly improved. 

May 1943 seems to have been the crucial month in 
the Battle of the Atlantic. The number of U-boats at 
sea in the Atlantic reached a peak of about 120, with 
about half of these U-boats concentrated in the 
Northwest Atlantic ^rea. The decisive battle took 
place at the beginning of the month between Convoy 
ONS 5 and a pack of about 40 U-boats. On May 4, 
when HF/DF activity indicated that the U-boats had 
made contact, the convoy consisted of about 30 ships 
and four stragglers. The average number of escorts 
present during the battle was about eight. Despite 
the bad weather, aircraft of the Royal Canadian Air 
Force provided air cover during the afternoon of the 
4th and carried out two promising attacks, one of 
which is considered to have sunk the U-boat. The 
U-boats started attacking shortly after midnight and 
12 ships were sunk on May 5, about half during the 


night and half during the next day. Due to bad 
weather, air cover was provided for only an hour on 
the morning of the 5th. By midnight the weather had 
become calm and foggy and from then onwards the 
escorts had the upper hand. In the course of the night 
they frustrated about 24 attacks by the U-boats with- 
out suffering any losses and, in addition, inflicted 
heavy losses and much damage on the enemy. At 
least five U-boats are considered to have been sunk 
by the escorts during the battle. The U-boats broke 
off the action in the course of the day and did not 
renew it. In no succeeding convoy operation did the 
enemy display the same determination. 

Several other convoys were threatened in the first 
half of the month but no serious losses were suffered 
and the toll of U-boats mounted steadily. After May 
17, no ship was lost in the Atlantic north of 45° north 
latitude. The world-wide shipping losses to U-boats 
in May 1943 decreased to 50 ships of 265,000 gross 
tons. Only 14 ships were sunk in the Northwest At- 
lantic Area while diversionary U-boats sank 15 ships 
in the Freetown and Southeast Atlantic Areas. 

The most notable feature of the operations during 
May was the success of our offensive against the 
U-boats. The number of U-boats sunk during the 
mondi reached a record high of 44, almost twice the 
previous high, with 38 of the sinkings occurring in 
the Atlantic. A record number of 15 were sunk in 
the Northwest Atlantic Area and the number sunk 
in the Bay of Biscay also reached a new high of 11. 
Shore-based aircraft accounted for over half the 
U-boats sunk during the month while carrier-based 
aircraft participated in three of the attacks in which 
the U-boat was considered to have been sunk. 

By the end of May it was clear that the U-boats had 
been decisively beaten as the number at sea in the 
Atlantic dropped to about 85, with no small propor- 
tion of the reduction due to sinkings of U-boats. The 
U-boats could not afford to suffer the heavy losses 
experienced in May and they were forced to with- 
draw from the battle against the vital North Atlantic 
convoys. This was an admission of defeat in itself. 

It should be noted that superior leadership and tac- 
tics, quick initial action, and well-coordinated attack 
and defense played as much of a part in the defeat of 
the U-boats as did concentration of forces at the de- 
cisive points and weapon superiority. 

By June 1943 the U-boats had retired from the 
main battlefield of the North Atlantic convoy routes 
and a ' considerable number had moved into the 


38 


LARGE WOLF PACKS BATTLE NORTH ATLANTIC CONVOYS 


Azores Area, outside the range of shore-based air- 
craft. As a result, June was an interim phase for the 
U-boats and they were able to sink only 20 ships of 
96,000 gross tons during the month, the smallest 
amount of shipping sunk by U-boats since November 
1941. Not a single ship was sunk in the Northwest 
Atlantic Area during June and only seven ships were 
sunk by U-boats in the whole Atlantic. Seven ships 
were sunk in the Indian Ocean and six in the 
Mediterranean. 

The offensive against the U-boats continued 
throughout June, but as the opportunities to attack 
U-boats were rarer due to their redisposition, only 
19 were sunk during the month. Aircraft continued 
to play an outstanding part, sinking ten U-boats, two 
by carrier-based aircraft from the USS Bogue in re- 
gions where the U-boats thought they would be safe 
from shore-based aircraft. The 2nd Escort Group led 
by HMS Starling sank three U-boats during the 
month and clearly demonstrated that a well-trained 
escort group could deal effectively with a U-boat. 
On June 2, this escort group made contact with 
U-202. Twelve depth-charge attacks were made with- 
out positive evidence of success, so it was decided to 
hunt the U-boat until its batteries were exhausted 
and it had to surface. One ship maintained contact 
while the remainder formed a square patrol. Despite 
every maneuver and artifice, including the release of 
19 SBT’s, the U-boat was compelled to surface after 
contact had been firmly held for 14i/^ hours. She was 
immediately attacked by gunfire and the crew aban- 
doned ship. On June 24, the 2nd Escort Group, act- 
ing as a striking force in cooperation with aircraft 
engaged in the Bay offensive, destroyed two more 
U-boats in the space of 9 hours. 

As the period closed, it was apparent that the 
U-boats had been decisively defeated in the battle 
against the North Atlantic convoys. The Allies had 
meanwhile taken the initiative and joined battle in 
the approaches to the Bay of Biscay, through which 
all U-boats must pass to and from their bases. 

5 2 COUNTERMEASURES TO THE 

U-BOAT 

Convoys 

There were only a few minor changes in the con- 
voy system during this period. The landings in North 
Africa in November 1942 resulted in the temporary 


suspension of sailings of the Sierra Leone and Gibral- 
tar convoys for several months due to lack of escorts. 
Shortly after these landings new convoys were started 
between the United States and the Mediterranean 
(designated UG and GU), to supply Allied forces in 
North Africa. The U. S. Coastal Convoy System was 
extended to Brazil in December 1942 following in- 
creased U-boat activity in that area. 

There were, however, a number of extensive 
changes in the routing of independent shipping. At 
the beginning of this period, the Allies were forced, 
by a concentration of U-boats between Natal and 
Dakar, to discontinue the route to the Red Sea and 
India via the Atlantic and the Cape of Good Hope. 
Shipping was instead routed via the Panama Canal 
and Cape Horn to Capetown. However, after the 
U-boats had started operations in force around Cape- 
town, the bulk of the Indian Ocean traffic was routed 
transpacific from Balboa south of New Zealand to 
Fremantle for onward routing to destination. This 
route through the South Pacific proved entirely safe. 
There have been losses in the Indian Ocean due to 
raiders and U-boats but, in general, this transpacific 
route was so successful that it was maintained until 
the Mediterranean was considered open in July 1943. 
It can be seen that there was a considerable gain in 
effective shipping due to the use of the short Medi- 
terranean route instead of the long circuitous route 
across the Pacific. 

During this period, the main efforts of the U-boats 
were concentrated against convoyed shipping, in par- 
ticular against the North Atlantic Trade Convoys 
(ON, ONS, HX, and SC). The proportion of tonnage 
sunk by U-boats, which was in convoy when sunk, in- 
creased from about 30 per cent during the last three 
months of 1942 to 67 per cent in March 1943 and 
then dropped back to about 25 per cent in June 1943 
after the U-boats had withdrawn from the North 
Atlantic convoy routes. About 720 ships sailed 
monthly in the North Atlantic Trade Convoys and 
about 21 of these ships were sunk monthly by 
U-boats, so that the loss rate was about 3 per cent per 
crossing. 74iis loss rate reached a peak of about 5 per 
cent in March 1943. By June 1943 the U-boats had 
been forced to withdraw and 850 ships arrived safely 
in these convoys without the loss of a single ship to 
the U-boats. 

The strategy of the U-boats in their battle against 
the North Atlantic convoys was to maintain patrols 
in positions designed to find convoys at a time when 




COUNTERMEASURES TO THE U-BOAT 


39 


they were about to leave the protection of air cover, pack is gathering on a trailed and reported convoy 
and It was in this gap that the U-boats scored their protective sweeps far ahead and to the flanks of the 
greatest success.There were three main U-boat forma- convoy enable aircraft to make killing and damaging 
tions for attacks on convoys, namely (1) Patrol line, attacks or at least to force U-boats to remain sub- 
(2) Reconnaissance sweep, and (3) Attack formation, merged at slow speed sufficiently long to delay the 
In the first, U-boats up to perhaps 25 in number are formation of an effective pack. Second, through keep- 
spread about 20 miles apart on a given line of bearing ing the trailing U-boats, on which the others are 
across the convoy routes. Each U-boat patrols at slow homing, submerged, aircraft can break the enemy’s 
speed on either side of and at right angles to the line, contact, enabling the convoy to escape by a change 
not going further than half an hour’s run from it. If of course. 

the U-boat sights a convoy she must report it to What the U. S. Navy needed in late 1942 was more 
Admiral, U-boats, who passes the signal to the other long-range bomber coverage. The U S Army was 
U-boats in the line. This Patrol line formation is a anxious to help and set up the 1st Anti-Submarine 
pool from which groups of U-boats may be detached Army Air Command in October 1942. This was an 
for any given duty, such as an attack on a convoy or a expansion of the 1st Bomber Command. Two anti- 
Reconnaissance sweep of an area through which a submarine squadrons were sent to England in De- 
convoy is expected to pass. cember 1942. Early in February 1943 they operated 

lor the Reconnaissance sweep formation, Admiral, in the Bay of Biscay and in March 1943 the two 
U-boats, signals the limits of the area to be swept, the squadrons were ordered to Port Lyautey, Morocco 
time at which the sweep is to start, and the U-boats where they were to help the U. S. Navy protect the 
which are to take part. The U-boats ordered to the Mediterranean Approaches. By the end of this period 
area, the limits of which may be 150 miles apart, pro- two other U. S. Army squadrons had been sent to 
ceed at a speed of about 10 knots on parallel courses England while two U. S. Navy squadrons were oper- 
and spread about 25 miles apart. When one of the ating from Iceland under Coastal Command 
U-boats has established the position of the convoy, a When the U-boats started using GSR at the begin- 
groiip of U-boats are ordered by Admiral, U-boats, to ning of this period in October 1942 the need for 
take up an attack formation. These U-boats may be microwave radar became more urgent As a stop-gap 
formed up into a semicircle around the line of ap- 14 DMS-lOOO’s were crash-built at a U. S. laboratory 
proach of the convoy. Once the order to attack is and rushed into British service. By the end of 1942 
given, there is no such thing as coordinated action the U. S. Navy’s AN/APS-2 [ASG] radar (S-band) 
between the U-boats, though they may keep in touch was coming into service. The U. S. Army equivalent 
with each other by radio. Each U-boat attacks at its was the SCR-517. 

discretion. The U-boats continue their attacks until The first effect of the introduction of GSR was a 
ordered by Admiral, U-boats, to break off. sharp reduction in the effectiveness of the aircraft 

An analysis of attacks by U-boats on the North offensive in the Bay of Biscay. The U-boats could 
Atlantic convoys at about the beginning of this pe- detect radar-fitted aircraft ranges which allowed 

nod indicated that about 70 per, cent of the at- them ample time to dive and the number of sightings 

tempted attacks were frustrated by the escorts. About and attacks on U-boats dropped sharply. This lull 
two ships were torpedoed in each successful attack, lasted through January 1943. Only four sightings per 
In only about half the successful attacks did the 1000 hours on patrol were made during this four- 

U-boat escape detection. Radar was responsible for month period, as compared to nine during the pre- 

more than half the cases in which the U-boat was vious four-month period, when the number of U-boat 
detected. . transits was actually smaller. The search receiver for 

Mark II ASV enabled the U-boats to make safe night 
Aircraft passages across the Bay and only 13 per cent of the 

. . . transits were sighted. No U-boat is considered to have 

The primary function of aircraft in the defense of been sunk by any of the attacks made in the Bay dur- 
convoys was the prevention of the gathering of ing this period. 

U-boat packs capable of making destructive attacks. This situation was changed by the introduction of 
I his function is accomplished in two ways. First, as a Mark III ASV (S-band) on Allied aircraft in Febru- 


40 


LARGE WOLF PACKS BATTLE NORTH ATLANTIC CONVOYS 


ary and March 1943. This set operated on a much 
shorter wavelength (10 cm) and could not be de- 
tected by the R-600 GSR. The proportion of aircraft 
approaches undetected by U-boats rose and conse- 
quently the sightings and attacks began a steady in- 
crease. This period of increased productivity in the 
Bay offensive lasted through July 1943. About 5500 
hours were flown monthly during this 6-month pe- 
riod with about 61 sightings and 33 attacks made 
monthly. About five U-boats were sunk monthly as a 
result of this offensive. The average number of sight- 
ings per 1000 hours on patrol jumped to 1 1 and about 
60 per cent of all transits were sighted during this 
period. 

The German High Command had no idea as to 
what caused this huge increase in sightings and at- 
tacks and after running into several blind alleys in 
attempting to solve the problem, they were forced to 
make a number of changes in U-boat tactics. The 
first step was the strengthening of the antiaircraft 
armament of U-boats. This development first became 
apparent in April 1943 when on a number of occa- 
sions U-boats, sighted by aircraft, stayed on the sur- 
face and fought back using their antiaircraft guns. 
Although a number of aircraft were lost as a result 
of this measure, it did provide aircraft with a much 
larger proportion of surfaced targets^and increased 
their chances of sinking a U-boat. Four U-boats that 
stayed up and fought back were sunk in April and 
nine such U-boats were sunk in May. 

During the spring of 1943, the night-flying Wel- 
lingtons, equipped with Leigh Lights and Mark Iff 
ASV, made night surfacing in the Bay so hazardous 
for the U-boats that they changed their policy to one 
of surfacing in the daytime to charge batteries and 
renew their air supply. This, in turn, produced the 
period of greatest productivity in the campaign dur- 
ing May, June, and July 1943, with the result that 
during the last two of these months every U-boat 
transit was sighted once on the average. May 1943 
was a record month for Coastal Command aircraft 
with the squadrons in the United Kingdom, Iceland, 
and Gibraltar accounting for 213 sightings and 136 
attacks, 17 of which are considered to have been 
lethal. One of the U-boats sunk in May was the victim 
of the first rocket attack against U-boats. This attack 
was made by a Swordfish aircraft. 

The next indication of a change in U-boat tactics 
was the sighting of five U-boats making the transit 
of the Bay in company on June 12. From then on- 


wards most of the U-boats sighted were in packs of 
from three to five. Although this change should have 
no effect on the number of U-boats sighted, the num- 
ber attacked will be less as each aircraft can normally 
only attack one U-boat. This effect appeared in June 
when there were 60 sightings in the Bay but only 28 
attacks. The other advantages of this change, for the 
U-boats, are that they will have a better chance of 
seeing aircraft, they will have much stronger anti- 
aircraft fire support, and it will be easier to provide 
fighter protection for them. Admiralty reacted im- 
mediately to this measure by sending surface craft 
hunting groups into the Bay to cooperate with air- 
craft and to follow up aircraft attacks. If the surface 
craft arrive at the scene of the attack quickly enough, 
several U-boats will be pinned down and the search 
should be easier. This countermeasure resulted in 
two of the U-boat kills made by the Second Escort 
Group in June. In addition. Mosquito fighters were 
sent into the Bay in June to operate against German 
aircraft covering the U-boats. 

Aircraft finally came into their own as an offensive 
power during this period. The 25-foot depth setting 
was in general use and, especially during the later 
months, planes were able to attack U-boats still on 
the surface. About 60 aircraft attacks were made 
monthly on U-boats during this period, with about 
25 per cent of the attacks resulting in at least some 
damage to the U-boat, while over 10 per cent of the 
attacks proved lethal. In addition aircraft were used 
to bomb U-boat bases and lay mines near them. 
These operations dislocated the servicing facilities at 
the bases and increased the turn-around time of 
U-boats operating from these bases. 

Scientific and Technical 

The main scientific battle during this period con- 
tinued to center about radar. As we have seen, the 
Metox R-600 GSR was able to detect Allied meter- 
wave radar. The Allied countermeasure to GSR was 
the introduction of short-wave (S-band 10-cm) radar 
about February 1943. This was made possible by the 
British invention of the strapped magnetron in the 
spring of 1940 which enabled sufficient power to be 
produced to make use of the shorter wavelengths 
practical. The Metox GSR was particularly unsuited 
to detecting these short wavelength radar transmis- 
sions and the number of aircraft attacks on U-boats 
increased sharply. The Germans had no idea of what 




SURVEY OF RESULTS 


41 


was causing the trouble and, suspecting supersonic 
modulation, they fitted GSR sets with a visual tuning 
indicator of the Magic Eye type. As this did not prove 
to be of any help they explored other blind alleys, 
suspecting infrared detection or intermittent opera- 
tion of radar sets, without hnding any solution dur- 
ing this period. The best the Germans could do was 
the fitting of a permanent GSR aerial, which did not 
have to be dismounted before diving, on some 
U-boats. Meanwhile it was evident in May and June 
1943 that there was a progressive lessening of con- 
fidence in GSR on German U-boats. Radar was fitted 
on some U-boats, apparently primarily as an offen- 
sive weapon against shipping in low visibility. How- 
ever, there was a great reluctance to use radar for 
fear of being detected by a hypothetical Allied search 
receiver. Radar decoy balloons were also used by 
some U-boats in an attempt to produce a large num- 
ber of false targets. 

A similar lack of appreciation of the situation was 
apparent in the U-boat’s attitude toward HF/DF. As 
the new technique of using shipborne HF/DF was 
learned and applied, the successes became more fre- 
quent, and by November 1942 HF/DF was accepted 
as an essential part of the equipment of escort craft. 
The U-boats overrated the accuracy of shore-based 
HF/DF, but for a long time they underrated, indeed 
ignored, the danger of shipborne HF /DF. This was 
reflected in their communications, which seemed to 
be conducted on the principle that radio silence was 
to be strictly kept until contact was made with the 
convoy, but completely relaxed once contact was 
made. In other words, they were afraid of revealing 
their dispositions but saw no danger from the trans- 
missions of individual U-boats once the battle was 
joined. During the attacks on Convoy SC 118 in 
February 1943, for example, the U-boats concerned 
made 108 transmissions during a period of 72 hours. 

During this period several ships were saved by 
AND, the torpedoes either being stopped by the nets 
or exploding in them. There were six successful 
Hedgehog attacks during this period and it appeared 
that the numerous early difficulties with this weapon 
had been overcome and it was beginning to show 
some signs of its theoretical lethality. 

It had become apparent in 1942 that some device 
was needed to enable attacking ships to maintain 
contact with deep U-boats to shorter distances. To 
meet this need the Q attachment was developed. It 
consisted of a small special sonar projector of high 


frequency, attached to the main projector below it 
and tilted down about 15°. Trials during this period 
indicated that good echoes could be obtained from 
targets within an angle of 45° from the horizontal. 

The expendable radio sono-buoy was developed 
during this period to enable aircraft to maintain 
contact with a submerged U-boat. It could be 
dropped from an aircraft and contained a hydro- 
phone which would listen to U-boat noises and a 
radio transmitter which would transmit the noises 
received to the plane. The service life of a sono-buoy 
was about four hours. 

Sinkings of U-boats 

The number of U-boats sunk or probably sunk 
during this critical period averaged about 19 a 
month, hitting a peak of 44 in May 1943. The total 
number sunk during this period was 168, more than 
twice the number sunk in any previous period. Of 
these, 128 were German, 18 Italian, 18 Japanese, and 
four were Vichy French. 

There were 112 U-boats sunk in the Atlantic dur- 
ing this period, 43 of these in the Northwest Atlantic 
Area and 25 in the Bay of Biscay. There were 37 
U-boats sunk in the Mediterranean, 15 during No- 
vember 1942, the month of the North African land- 
ings. The 18 Japanese U-boats were all sunk in the 
Pacific. 

This was the first period of the war in which air- 
craft were the leading killers of U-boats, sinking 76 
U-boats (45 per cent of the total) alone and another 
10 (6 per cent) in cooperation with surface craft. Ten 
of these 86 successful aircraft attacks involved carrier- 
based aircraft. Ships accounted for 59 U-boats (35 
per cent) and submarines for 17 (10 per cent). 

The quality of surface craft attacks continued to 
improve. About 60 attacks were made monthly on 
U-boats in the Atlantic and Mediterranean and 
about 25 per cent of these surface craft attacks re- 
sulted in at least some damage to the U-boats, while 
10 per cent of these attacks resulted in the sinking of 
the U-boat. 

SURVEY OF RESULTS 

5.3.1 From the U-boat’s Point of View 

This was the crucial period of the war for the 
U-boats, the one in which they made their supreme 
effort and were decisively defeated. The world-wide 
shipping losses to U-boats were about 30 per cent 


42 


LARGE WOLF PACKS BATTLE NORTH ATLANTIC CONVOYS 


lower than during the preceding period, as about 67 
ships of 394,000 gross tons were sunk monthly by 
U-boats, with about 85 per cent of these losses oc- 
curring in the Atlantic. The number of U-boats sunk 
throughout the world was about 19 a month, more 
than twice as many as during the previous period. 
The world-wide exchange rate was, therefore, only 
about 31/2 ships of 21,000 gross tons sunk by the 
average U-boat before it, itself, was sunk. This ex- 
change rate was only one-third as large as it had 
been during the previous period. 

Activity in the Mediterranean was greatly in- 
creased as 44 ships were sunk by U-boats and 37 
U-boats were sunk. U-boat operations in the Medi- 
terranean were, in many cases, auxiliary to the mili- 
tary operations in North Africa. In the Indian Ocean, 
German and Japanese U-boats sank 25 ships, half 
the total in the previous period, without suffering 
any losses. Japanese U-boats sank 19 ships in the 
Pacific and suffered the loss of 18 U-boats, an ex- 
tremely unprofitable return. 

In the Atlantic, the scene of the main battle, the 
average number of U-boats at sea during this period 
was 104, higher than in any other period of the war. 
These U-boats sank about 57 ships of 344,000 gross 
tons monthly, about one-third of these sinkings oc- 
curring in the Northwest Atlantic Area. This meant 
that the efficiency of U-boats reached a new low for 
the war as the average U-boat sank only about one- 
half ship of 3200 gross tons per month at sea, only 
about one-third the results achieved in the previous 
period. This indicated that the U-boats’ campaign 
of wolf-pack attacks against the North Atlantic con- 
voys had failed to maintain the average U-boat ef- 
fectiveness at its previous high level. The U-boats 
were very rarely able to completely overwhelm the 
escorts of the convoys and consequently the use of 
large wolf packs of over 20 U-boats proved relatively 
inefficient. In addition, when the U-boats had to 
operate against convoys which were given air cover- 
age, their wolf-pack operations broke down com- 
pletely as they depended upon surfaced U-boats to 
keep contact with the convoy and to home the other 
U-boats. 

In addition to the reduction in its efficiency, the 
average U-boat in the Atlantic found life more haz- 
ardous during this period than in the preceding 
period. This marked the first time in the war that the 
trend of increasing safety for the U-boat was reversed. 
About 121/2 U-boats were sunk monthly of the 104 


U-boats at sea in the Atlantic, so that the average life 
of a U-boat at sea in the Atlantic during this period 
was about 81/2 months, about one-third less than in 
the previous period. The main factors causing this 
shorter average life for U-boats were the increased 
effectiveness of Allied aircraft and the fact that the 
U-boats were exposing themselves to increased at- 
tacks by surface escorts by concentrating against con- 
voyed shipping. 

These figures indicate that the average U-boat in 
the Atlantic during this period was sinking only 
about 41/2 ships of 28,000 gross tons before it itself 
was sunk. This exchange rate was the lowest of the 
war to that date and only one-fourth as high as the 
record exchange rate achieved in the previous period. 
This low exchange rate plus the fact that, in the 
Atlantic, the number of U-boats sunk in May 1943 
was almost as large as the number of ships sunk by 
U-boats, must have convinced the Germans that they 
could not continue the battle against the North At- 
lantic convoys and they disengaged. The average 
number of U-boats at sea in the Northwest Atlantic 
Area dropped from 60 in May to 20 in June 1943. 

The expansion of the German U-boat fleet was 
definitely slowed down during this period. Bombing 
of construction yards had kept new construction at 
about the same level as in the previous period, with 
about 20 new U-boats being commissioned monthly. 
About 128 German U-boats are considered to have 
been sunk during this period and a number of others 
decommissioned. Consequently, the available num- 
ber of German U-boats increased from about 350 at 
the beginning of this period to about 400 at the end 
of the period. This huge fleet still constituted a con- 
siderable potential threat, but many experienced 
officers and crews had been lost during this period, 
and the heavy losses suffered by the U-boats must 
have had an adverse effect on enemy morale. 

5.3.2 From the Allies’ Point of View 

Total shipping losses of the Allied and neutral 
nations were about 491,000 gross tons a month dur- 
ing this period, about 30 per cent less than during the 
preceding period. Construction of new merchant 
shipping reached a new high of about 1,026,000 gross 
tons a month, more than twice the total monthly 
losses. Consequently, the net monthly gain of ship- 
ping was about 535,000 gross tons. This was the first 
period of the war in which new construction of ship- 


SURVEY OF RESULTS 


43 


ping exceeded the total losses. The total shipping 
available increased by almost 5,000,000 gross tons 
during this period to a level of about 36,500,000 gross 
tons. It appeared that the shipping crisis in the war 
had definitely passed, as it seemed extremely unlikely 
that the shipping losses would ever exceed the mil- 
lion gross tons being constructed monthly. The main 
problem facing the Allies at the end of this period 
was to keep the shipping losses down to a minimum 
so that supplies could be built up quickly in England 
for the invasion of the continent. 

Of the 491,000 gross tons of shipping lost monthly, 
about 436,000 gross tons were lost as a result of enemy 
action. U-boats accounted for 394,000 gross tons a 
month, about 90 per cent of the total lost by enemy 


action. Losses to enemy aircraft, surface craft, and 
mines were all considerably lower during this period 
than in the preceding one. 

The number of ships suitable for ocean escort, 
available to the Allies, had increased from about 745 
at the beginning of this period to about 950 at the 
end of June 1943. This included about 50 new de- 
stroyer escorts [DE] as well as about 25 new escort 
carriers [CVE]. This increased number of escorts and 
the new VLR aircraft that had become available 
placed the Allies, at the end of this period, in the 
position of having defeated the greatest efforts the 
U-boats could make. The Allies now had the oppor- 
tunity of taking the offensive against the U-boats to 
prevent them from ever regaining the initiative. 


Chapter 6 

SIXTH PERIOD 

AIRCRAFT DEFEAT U-BOATS’ ATTEMPTED COME-BACK AND 
FORCE ADOPTION OF MAXIMUM SUBMERGENCE 
JULY 1943-MAY 1944 


6.1 U-BOAT OFFENSIVE 

As A RESULT of the heavy losses they experienced in 
their battle against the North Atlantic convoys 
during the previous period the U-boats withdrew 
from the North Atlantic and temporarily abandoned 
their wolf-pack tactics. After a marked lull in U-boat 
activity in June, the enemy made his first, and most 
strenuous, effort to regain the initiative in July 1943. 
The 85 U-boats at sea in the Atlantic were widely 
dispersed and simultaneous campaigns were con- 
ducted in regions that had been soft spots for the 
U-boats in the past, such as the Caribbean, Brazilian, 
and Freetown Areas. To withdraw from the stra- 
tegically important North Atlantic was a confession 
of defeat, but the enemy must have anticipated a 
rich harvest from surprise attacks in comparatively 
lightly protected areas. 

If so, the enemy was probably sadly disappointed. 
World-wide shipping losses as a result of U-boat ac- 
tion in July were only 44 ships of 244,000 gross tons, 
considerably less than the monthly average during 
the preceding period. The heaviest losses occurred 
in the Indian Ocean, mostly in the Mozambique- 
Madagascar Area, where 15 ships of 82,000 gross tons 
were sunk by U-boats while no U-boats were sunk. 
This was the highest monthly total of the war for 
shipping losses to U-boats in the Indian Ocean. 
About six German U-boats of the 1200-ton U-Kreuzer 
class were probably responsible for most of these 
sinkings. 

About 15 U-boats operated in the Caribbean Sea 
and off the Brazilian coast as far south as Rio de 
Janeiro. They sank 19 ships during July, but were 
forced to pay a heavy price for these successes as nine 
U-boats were sunk by shore-based aircraft in these 
areas. This eliminated another soft spot from the list 
of areas where U-boats felt they could operate safely. 

July 1943 was marked by the successful invasion 
of Sicily, which took place without the loss of any 


shipping to U-boats. Only five ships were sunk by 
U-boats in all of the Mediterranean at the cost of 
seven U-boats sunk by the Allies. 

Despite the meager results achieved, the U-boats 
suffered the heaviest losses of the war in July 1943 as 
the world-wide total of U-boats destroyed during the 
month reached 46. This was the first month of the 
war in which the number of U-boats sunk exceeded 
the number of merchant vessels sunk by U-boats. Of 
the 46 U-boats sunk during July, 37 were destroyed 
in the Atlantic where aircraft experienced their 
greatest successes of the war. Shore-based aircraft 
destroyed 28 U-boats in the Atlantic while carrier- 
based aircraft accounted for another six. The aircraft 
from the U. S. escort carriers operated in regions out- 
side the range of shore-based aircraft and were able 
to prevent the U-boats from achieving any successes 
in their attacks against the mid-Atlantic convoys. 
These attacks by carrier-based aircraft must have 
finally convinced the U-boats that they were no 
longer safe from air attack anywhere in the Atlantic. 

Allied antisubmarine forces inflicted the greatest 
damage on the enemy in the Bay of Biscay and its 
approaches as 14 U-boats were sunk in the Biscay- 
Channel Area and another six in the Gibraltar- 
Morocco Area. Although aircraft crews had to face 
the increased antiaircraft fire of surfaced U-boats 
proceeding in formation during the daytime, this 
presented them with a large proportion of Class A 
targets and over 25 per cent of the attacks resulted in 
the sinking of the U-boat. The crowning success of 
the month occurred on July 30 when a whole group 
of three outward bound U-boats was sunk, two by 
Coastal Command aircraft and the third by the Sec- 
ond Escort Group. Two of these three U-boats were 
supply U-boats, and this plus the loss of two other 
supply U-boats elsewhere severely curtailed later 
U-boat operations. This incident was also marked by 
one of the odd coincidences of the war when U-461 
was sunk by the Sunderland aircraft U/461. 


U-BOAT OFFENSIVE 


45 


Four additional U-boats were sunk in the Bay of 
Biscay by Coastal Command aircraft during the first 
two days of August, and the U-boats were forced to 
change their tactics in making the transit of the Bay. 
They reverted to surfacing at night for the minimum 
time necessary for the charging of batteries and, in 
addition, hugged the coast of Spain to get as far as 
possible from Allied air bases. 

The more cautious tactics adopted throughout the 
Atlantic in August 1943 marked the failure of the 
first attempted come-back by the U-boats. Only four 
ships were sunk by U-boats in the Atlantic during 
the month, all during the first week of August and 
all south of the Equator. The world wide shipping 
losses to U-boats in August were only 15 ships of 
87,000 gross tons as the U-boats sank six ships in the 
Indian Ocean and five in the Mediterranean. 

The average number of U-boats at sea in the Atlan- 
tic during August declined to about 60, and most of 
them were homeward bound by the end of the 
month. There was no doubt that, as a direct conse- 
quence of the loss of a number of supply U-boats 
and also of the heavy losses suffered by U-boats in 
the operating areas, the campaigns in the Caribbean 
and Brazilian Areas were curtailed, and the U-boats 
returned to base about two weeks earlier than they 
would otherwise have done. 

The number of U-boats sunk during August con- 
tinued to be satisfactory, considering the smaller 
number of targets available and the more cautious 
tactics adopted by the U-boats. Aircraft attacks ac- 
counted for 18 of the 25 U-boats sunk throughout the 
world. Aircraft from the USS Card turned in a partic- 
ularly notable performance by sinking five U-boats 
during the month. New evidence of the gradual dis- 
appearance of all soft spots was indicated by the 
sinking of three U-boats in the Caribbean-Brazilian 
Area and two U-boats in the Freetown Area. One 
U-boat was also sunk in the Indian Ocean, the first 
in 1943. 

As the lull in U-boat activity continued during 
August and the first three weeks of September, the 
Allies attempted to maintain the initiative by carry- 
ing the battle into the Biscay transit areas. Escort 
groups were moved closer to the Spanish Coast in an 
effort to sever the new U-boat routes. However, Ger- 
many reacted strongly to this new threat, bringing 
out a new weapon, the radio-controlled, jet-propelled 
glider bomb. These were released by German bomb- 
ers against the escort groups in the Bay. HMS Egret 


was sunk by this new weapon late in August and two 
other ships were damaged. The escort groups were 
withdrawn from the Bay offensive in September, as 
a result of these attacks and renewed U-boat activity 
in the North Atlantic. 

This resumption of the battle against the vital 
North Atlantic convoys marked the second attempt 
of the U-boats to regain the initiative. It was on Sep- 
tember 19, 1943, that it first became evident that Con- 
voys ONS 18 and ON 202 were being shadowed by 
U-boats. That night, attacks developed against both 
convoys, but the escorts of ONS 18 were able to drive 
the U-boats off without suffering any losses. 

Convoy ON 202 was attacked later that night and 
HMS Lagan was torpedoed. This was the first ship 
torpedoed in the Atlantic in September. It was also 
the first indication that the U-boats were using a new 
weapon, the acoustic homing torpedo (Gnat or T-5). 
HMS Lagan had been detached to follow up an 
bearing and had obtained a radar contact 
which faded when the range was about 3000 yards. 
She was within about 1200 yards of the assumed div- 
ing position when she was hit by a torpedo which 
blew off her stern. She was taken in tow and reached 
harbor. In the morning two merchant ships of Con- 
voy ON 202 were also torpedoed. 

During the forenoon the two convoys were ordered 
to join, forming one convoy of 63 ships and about 15 
escorts. That evening two escorts were sunk, one 
while hunting a U-boat, the other while following up 
a radar contact. Despite these losses, the attempts 
which the U-boats made on the convoy during the 
night failed. Early on the 21st, the convoy was re- 
routed to the southward in order to get within range 
of Newfoundland-based aircraft. However, heavy 
fog prevented much air cover that day and also ham- 
pered the U-boats. The U-boats, estimated to be 15 
or 20 in number, had the worst of it that night. 
Thanks to HF/DF fixes by the escorts, the U-boats 
were prevented from sinking any ships and in the 
early hours of the 22nd, one of the pack was rammed 
and sent to the bottom by HMS Keppel. 

Liberators gave cover throughout the day but the 
convoy was rather opened up by night and the 
U-boats again attacked in strength. Just before mid- 
night one escort was sunk, later three merchant ves- 
sels were torpedoed and one more ship was sunk early 
that morning. The U-boats kept in contact with the 
convoy during the 23rd, but the attacks made by 
escorting Liberators from Newfoundland so deterred 


46 


AIRCRAFT DEFEAT U-BOATS’ ATTEMPTED COME-BACK 



Figure 1. Bombs explode close aboard in attack by aircraft from USS Card, August 7, 1943, off the Azores. Note anti- 
aircraft guns. 


them that their attacks that night were halfhearted 
and they lost contact with the convoy next day. 

In all, six merchant vessels and three escorts had 
been sunk, and another escort damaged. Three 
U-boats were sunk, two by covering aircraft and one 
by an escort. Although heavy losses were inflicted on 
the escorts by the acoustic torpedo, on only three 
occasions did U-boats succeed in firing torpedoes at 
the convoy. The U-boats were more reluctant then 
ever to press home their attacks, and as soon as they 
were detected, they made every effort to escape, either 
at high speed on the surface or by going deep. 
HF /DF was again of great value in determining the 
direction of impending attacks. 

The actual results of this battle would probably 
have proved disappointing to the enemy but cap- 
tured documents indicate that the U-boats greatly 


overestimated the damage inflicted by the acoustic 
torpedoes. In addition, they felt that the heavy fog 
was largely responsible for saving the convoy from 
much heavier losses. This probably accounts for the 
greatly increased effort they made against the North 
Atlantic convoys in October 1943. 

The world-wide shipping losses to U-boats in Sep- 
tember were 20 ships of 119,000 gross tons, only 
slightly higher than the August losses. In addition to 
the six ships sunk in the North Atlantic, two were 
sunk in the Brazilian Area, six in the Mediterranean 
and six in the Indian Ocean. About five German 
U-boats appeared to be responsible for the sinkings 
in the Indian Ocean and there was evidence that 
they were using Penang as a temporary base. The 
more cautious tactics of maximum submergence 
served to reduce the number of U-boats sunk during 




U-BOAT OFFENSIVE 


47 



77 * “’’■P '^y USS Card, August 9, 

the stern, and life-rafts are visible at the right. ° 


1943; U-boat is settling 1 


September to only 10. By the end of September 1943, 
however, the Italian Fleet had surrendered, and 29 
Italian U-boats had come under Allied control. 

Encouraged by their supposed successes in Sep- 
tember, the U-boats increased their attack against the 
North Atlantic convoys. The downward trend in the 
number of U-boats at sea was reversed, and there 
were about 75 U-boats at sea in the Atlantic in Octo- 
ber as compared to only about 50 in September. Over 
40 U-boats were concentrated in the Northwest and 
Northeast Atlantic Areas and the days of wolf-pack 
attacks on convoys seemed to have returned. 

However, this time the U-boats suffered a much 
more decisive defeat than the one they had experi- 


enced in May 1943. First of all, the U-boats had 
trouble in locating Allied convoys because of evasive 
routing, and they were able to inflict losses on only 
three of the North Atlantic convoys during October. 
Then again, the U-boats were not attacking with the 
same aggressiveness they had shown in the past, and 
they were able to sink only three merchant vessels 
and one escort. Moreover, the U-boats had to pay the 
prohibitive price of over seven U-boats sunk for each 
merchant vessel sunk as 22 U-boats were sunk during 
October in the Northwest and Northeast Atlantic 
Areas. Aircraft played a major part in this decisive 
setback of the U-boats destroying 17 of the 22 U-boats 
sunk in these areas. Carrier-based aircraft accounted 








48 


AIRCRAFT DEFEAT U-BOATS’ ATTEMPTED COME-BACK 


for six of these 17 U-boats with four of the kills due to 
aircraft from the USS Card. In two cruises, aircraft 
from the USS Card had made 19 attacks, nine of 
which resulted in A or B assessments. This second 
cruise closed with the gallant action in which the 
USS Borie, one of the escorts of the USS Card, was 
lost. 

On the night of October 31, after having severely 
damaged one U-boat reported by a plane from the 
USS Card, the USS Borie made radar contact with 
another U-boat. Sonar contact was obtained and a 
depth-charge attack brought the U-boat to the sur- 
face. The U-boat tried to escape on the surface, but 
the Borie opened fire and then rammed the U-boat 
at 25 knots, riding over the U-boat’s forecastle and 
pinning it under. The two ships remained in this 
position for about 10 minutes with the U-boat ex- 
posed to fire from the Borie' s 4-inch and 20-mm guns. 
Tommy guns, revolvers, rifles, and shotguns. One of 
the U-boat’s crew was killed by a sheath knife thrown 
from the Borie's deck while another was knocked 
overboard by an empty 4-inch shell case. At length, 
the U-boat got under way, with the Borie, now se- 
verely damaged, in full pursuit maintaining gunfire. 
A torpedo was fired at the U-boat but missed. The 
range was then closed again and, as the attempt to 
ram failed, three depth charges were fired straddling 
the U-boat. Another torpedo was then fired passing 
within 10 feet of the U-boat’s bow. A main battery 
salvo struck the U-boat’s diesel exhaust and the 
U-boat immediately slowed, stopped, and surren- 
dered. About 15 members of the crew abandoned 
ship and the U-boat sank stern first. The entire action 
from the initial contact until the U-boat sank lasted 
one hour and four minutes. Unfortunately, the ram- 
ming resulted in serious damage to the Borie and 
she had to be abandoned later in the day. 

The world-wide shipping losses to U-boats in Oc- 
tober were about the same as in September. Only 20 
ships of 97,000 gross tons were sunk, but the number 
of U-boats sunk increased to 27 reflecting the in- 
creased U-boat activity. The U-boats were forced to 
disengage again in their battle against the North 
Atlantic convoys and their second attempt at a come- 
back had failed. The first promising results of the 
acoustic torpedo were not maintained, and during 
October this new weapon had singularly little suc- 
cess. By way of countermeasures the Allies developed 
a towed noise-making device (U. S. FXR— British 
FOXER) and new step-aside tactics in attacking 


U-boats. By the end of October the British had been 
granted the use of the Azores by Portugal, and air 
bases were immediately established greatly extending 
Allied air coverage of the Atlantic. 

A fundamental change in U-boat tactics was ob- 
served in November 1943 as the U-boats made their 
third, and most feeble, attempt to regain the initia- 
tive. As a result of the heavy losses inflicted on the 
U-boats by aircraft in October, and also influenced 
by the Allied air bases on the Azores, the U-boats 
were compelled to adopt a mode of existence which 
favored their survival rather than the most effective 
employment against shipping. In order to favor their 
chances of survival, the U-boats remained completely 
submerged during the daytime, thereby avoiding 
Allied air patrols. They surfaced at night to charge 
batteries and to follow-up and attack any convoys 
within reach. Small groups of U-boats were disposed 
along the probable course of the convoy, about one 
day’s run apart, so that each pack would be able to 
attack the convoy for only one night. To compensate 
for this loss of mobility of the U-boats, enemy long- 
range reconnaissance aircraft were used to locate our 
convoys and to pass on information to the U-boats. 

As most of the enemy’s long-range aircraft were 
based in France, these new U-boat tactics were tried 
out on the convoy route between the United King- 
dom and Gibraltar. Four north bound convoys were 
attacked by the U-boats with very little success, as 
only one ship was sunk while several U-boats were 
destroyed. Fortunately, the attacks on these convoys 
came after Allied aircraft had moved into the Azores 
and strong support by both escort groups and shore- 
based aircraft brought the convoys safely through 
the concentrations of U-boats. These attacks also pro- 
vided Azores-based aircraft with their first kill as a 
Flying Fortress sank a U-boat at daybreak on No- 
vember 9. 

After the failure of this third attempt at staging a 
come-back the U-boats reconciled themselves to re- 
maining on the defensive for the remainder of this 
period. The world-wide shipping losses to U-boats 
dropped to their lowest level since Pearl Harbor as 
only 13 ships of 67,000 gross tons were sunk during 
November 1943. There was a spurt of activity in the 
Panama Sea Frontier as three ships and one schooner 
were sunk by U-boats. Not a single ship was sunk in 
the North Atlantic Convoy Area during the month, 
while four ships were sunk in the Indian Ocean and 
three in the Mediterranean. German aircraft, mak- 




U-BOAT OFFENSIVE 


49 


ing extensive use of torpedoes and glider bombs, had 
become a greater menace to the Mediterranean con- 
voys than were the U-boats. Aircraft sank seven ships 
of over 60,000 gross tons in the Mediterranean dur- 
ing November, six of them off the coast of North 
Africa between Iran and Bizerte. Early in the month, 
the enemy’s attempt to reinforce the diminishing 
number of U-boats in the Mediterranean was largely 
frustrated as three of the five U-boats making the 
attempted entry were sunk. 

In all, 18 U-boats were sunk during November, 
about half by aircraft and half by surface craft. These 
U-boat kills were rather widely distributed. The 
Second Escort Group sank two U-boats using a 
creeping attack. This method of attack was devel- 
oped to take care of deep U-boats and involved one 
ship’s keeping contact with the deep U-boat at a dis- 
tance and directing the attacking ship onto the un- 
suspecting U-boat. The attacking ship proceeded 
silently at slow speed in order to surprise the U-boat 
before it could start any violent evasive maneuvers. 
When the attacking ship was over the U-boat, it 
would drop a large pattern of depth charges. 

Two large U-boats, apparently headed for the 
Indian Ocean, were sunk in the South Atlantic dur- 
ing November by U. S. aircraft based on Ascension 
Island. These sinkings were particularly valuable as 
the Indian Ocean was one of the few remaining areas 
where the exchange rate was still favorable to the 
U-boats. The sinking of a U-boat headed for the 
Indian Ocean might be considered equivalent to the 
saving of ten ships, which the average U-boat would 
probably sink in the Indian Ocean before it itself 
would be sunk. 

Barrier patrols by aircraft in the South Atlantic 
paid off during the last week of December 1943 and 
the first week of January 1944. Five enemy merchant 
vessels attempted to return from Japan to Germany 
and were in the South Atlantic heading north. One 
of the five enemy blockade runners made port, badly 
damaged, three were sunk by ships and aircraft of 
the Fourth Fleet in the Brazilian Area, and another 
was sunk by British-based aircraft in the approaches 
to the Bay of Biscay. This was the last attempt the 
enemy made to run the blockade with merchant 
vessels, but U-boats continued to make occasional 
trips between Germany and Japan. 

World-Wide shipping losses to U-boats stayed at 
the same low level as only 13 ships of 87,000 gross 
tons were sunk by U-boats in December 1943. Al- 


though over half of the 60 U-boats at sea in the At- 
lantic were concentrated in the North Atlantic Gon- 
voy Area, not a single ship was sunk there. All the 
sinking were due to small numbers of U-boats oper- 
ating in distant areas. Five ships were sunk in the 
Indian Ocean, three in the Freetown Area, and one 
in the Mediterranean. The other four ships were 
sunk in American coastal waters extending from 
Cape Flatteras to Aruba. However, the U-boats’ pol- 
icy of remaining submerged during the daytime re- 
sulted in only seven U-boats being sunk during De- 
cember, the lowest monthly total in 1943. 

The shipping losses to U-boats in December were 
overshadowed by the surprise air attack on Bari 
Harbor, on the east coast of Italy, on December 2. 
This attack, made by about 25 bombers, resulted in 
the loss of 16 ships and damage to 10 others. The 
heavy losses were due in part to the explosion of 
several ammunition ships. 

The U-boats sank only 13 ships of 92,000 gross 
tons in January 1944. Two of these ships were sunk 
from North Atlantic convoys. Three ships were sunk 
m the Barents Sea Area from a convoy headed for 
Russia while eight ships were sunk in the Indian 
Ocean. The main concentration of about 25 U-boats 
in the North Atlantic moved gradually eastwards 
towards the west coast of Ireland, in an apparent 
attempt to locate Allied convoys. This move did not 
succeed as no ships were sunk in the Northeast Atlan- 
tic Area while seven U-boats were destroyed there 
However, only 11 U-boats were sunk in January as 
the U-boats continued their cautious tactics of maxi- 
mum submergence. Fear of the impending invasion 
may have played its part in keeping the U-boats 
within short range of the French coast. 

February 1944 marked the fourth successive month 
m which fewer than 20 ships and less than 100,000 
gross tons of shipping were sunk by U-boats. Only 
18 ships of 93,000 gross tons were sunk during the 
month, ten of them in the Indian Ocean, six in the 
Mediterranean, and only two in the Atlantic, one 
near Iceland and one in the Freetown Area. About 
half of the 60 U-boats at sea in the Atlantic were 
concentrated in the North Atlantic, west of th- 
United Kingdom. However, the North Atlantic con- 
voys passed through this concentration of U-boats 
without suffering any losses while severe losses were 
inflicted on the U-boats. The world-wide total of 
U-boats sunk mounted to 22 in February, as ten 
were destroyed in the Northeast Atlantic Area. 


50 


AIRCRAFT DEFEAT U-BOATS’ ATTEMPTED COME-BACK 



Figure 3. Crew of U-550 prepare to abandon ship after attack by USS Joyce, Gandy, and Peterson, 200 miles off New 
York, April 16, 1944. SS Pan Pennsylvania burning in the background. 


The outstanding achievement of the month was 
the 27-day patrol of the Second Escort Group, in the 
course of which six U-boats were sunk in a period of 
20 days. Every U-boat contacted was hunted to de- 
struction. Four hours and 106 depth charges were 
required, on the average, to kill each of these six 
U-boats. Combined creeping and follow-up attacks 
were responsible for the destruction of four of them. 
Against these successes must be recorded the loss of 
HMS Woodpecker to an acoustic torpedo. In con- 
trast to these attacks, HMS Spey, fitted with the latest 
type of Asdic gear including the Q attachment and 
the Type 147B depth predictor, destroyed two 
U-boats in two days, each in a matter of minutes. 
Single patterns of ten depth charges forced the 
U-boat to the surface in each case. 

Another outstanding feature of the month’s opera- 
tions was the first sinking of a U-boat as the result of 
an initial MAD contact. U-boats had, during the pre- 
vious months, approached on the surface at night and 
passed through the Straits of Gibraltar in the day- 
time entirely submerged.They made only enough 
speed to maintain trim, while the current carried 
them through. Allied ships, operating in the Straits, 
faced bad sound conditions, because of the variation 
in the density of the water. Echo ranging was unre- 


liable and listening was not of much value either, as 
the U-boats proceeded at very slow speeds. 

This situation presented an ideal set-up for the 
use of MAD. An MAD barrier patrol was started 
across the Straits in January 1944, in order to prevent 
the submerged passage of U-boats into the Mediter- 
ranean. Two PBY’s of VP63 were flying on this patrol 
on February 24 when an MAD contact was obtained. 
Shortly afterward, two British destroyers and other 
planes arrived on the scene. The U-boat was attacked 
with retro-bombs from the MAD planes, depth 
charges and gunfire from the destroyers, and depth 
bombs from the other aircraft before it was de- 
stroyed. This MAD barrier patrol probably de- 
stroyed another U-boat attempting to make the pas- 
sage in March. 

During March 1944 there were some signs of the 
breaking up of the concentrations of U-boats in favor 
of a general dispersion across the North Atlantic 
convoy lanes. There was an increase in the shipping 
losses to U-boats as 23 ships of 143,000 gross tons 
were sunk throughout the world. Losses were again 
heaviest in the Indian Ocean where 11 ships were 
sunk, but the German U-boats operating there were 
seriously inconvenienced by the sinking in the South 
Indian Ocean of two tankers which had been refuel- 




U-BOAT OFFENSIVE 


51 


r' 



Figure 4. U-550 sinks. 


ing them. No ships were sunk in the Indian Ocean 
during the remaining two months of this period. 
Seven ships were sunk in widely scattered parts of 
the Atlantic in March, four in the Mediterranean, 
and one in the Barents Sea Area. 

The number of U-boats sunk increased for the 
third successive month as 23 U-boats were destroyed 
in March, the same as the number of ships sunk by 
U-boats. Two of the longest U-boat hunts of The-War 
occurred at about this time, one lasting 30 hours and 
the other 38 hours before the U-boats were destroyed. 
The Second Escort Group accounted for two more 
U-boats in March, bringing its total up to 14. One of 
these was sunk with the cooperation of aircraft from 
the escort carrier, HMS Vindex. 

The escort carrier groups contributed greatly to 
the campaign in March, sinking nine U-boats in the 
Atlantic. Aircraft from HMS Chaser, using rockets, 
participated in the sinking of three U-boats. Aircraft 
and escorts of the USS Bogue participated in the sink- 
ing of one U-boat while two other U-boats were sunk 
by escorts of the USS Block Island. Toward the latter 
part of the month, the USS Block Island task group, 
operating in a probable refueling area near the Cape 
Verde Islands, sank two more U-boats. Sono-buoys 
were used in this operation. 


In the Mediterranean, the enemy lost five U-boats 
out of the comparatively small number remaining 
there. Two of these were sunk in a raid by U. S. Army 
Liberators on the U-boat base at Toulon. The lack 
of sightings in the Bay of Biscay caused Coastal Com- 
mand to send its aircraft closer to the French coast 
and a U-boat was probably sunk by hits from 6- 
pounder gunfire from a Mosquito. A Catalina from 
Capetown sank a U-boat about 400 miles south of 
the Cape of Good Hope. 

The average number of U-boats at sea in the At- 
lantic dropped to 50 in April 1944 as the enemy con- 
tinued to conserve his U-boats in anticipation of a 
major effort against the threatened invasion. U-boat 
patrol dispositions were rather sparse and wide-flung 
and great importance seemed to be attached to the 
gathering of meteorological information, in aid of 
military and air planning. Only nine ships of 62,000 
gross tons were sunk by U-boats during the month, 
seven in the Atlantic and two in the Mediterranean. 

For the first time in 14 months, not a single ship 
was sunk by U-boats in the Indian Ocean. However, 
the most disastrous event of the month occurred 
there, when on April 14 an ammunition ship ex- 
ploded in Bombay harbor, completely destroying 14 
ships and damaging nine others. As partial com- 




52 


AIRCRAFT DEFEAT U-BOATS’ ATTEMPTED COME-BACK 


pensation for this disaster, the world-wide losses due 
to enemy action reached a new low for the war as 
only 13 ships of 82,000 gross tons were sunk in April. 

Starting in April, the U. S. ocean escorts of the 
UGS convoys continued past Gibraltar into the 
Mediterranean as far as Bizerte before being relieved 
by British escorts. Each of these three April convoys 
was subjected to the familiar Nazi air attack near 
Algiers as a result of which three ships were sunk and 
five damaged. 

On the night of April 20, the Germans made an 
unsuccessful attack with human torpedoes at Anzio, 
and both prisoners and craft were taken. These craft 
consisted of two torpedoes, one mounted about 6 
inches above the other. The “mother” (upper) tor- 
pedo was modified to hold a human pilot who guided 
the craft and released the normal lethal torpedo at- 
tached underneath when within firing range. 

Considering the inactivity of the U-boats, it was 
remarkable that 16 of them were sunk during April. 
The escort carrier groups accounted for six of these. 

The average number of U-boats at sea in the At- 
lantic declined to only about 40 in May 1944. How- 
ever, a large number of U-boats, which normally 
would have been operating in the Atlantic, was ap- 
parently being held in Biscay ports and in Baltic and 
Norwegian ports for use against the forthcoming in- 
vasions. World-wide shipping losses to U-boats were 
lower than they had ever been before as only four 
ships of 24,000 gross tons were sunk by U-boats dur- 
ing May. Three of these ships were sunk in the Bra- 
zilian Area and one in the Mediterranean. Total 
shipping losses from all causes also dropped to a new 
low as only 12 ships of 40,000 gross tons were lost 
during May. 

The most notable achievement of the U-boats dur- 
ing May was the sinking of the escort carrier, USS 
Block Island. Early in the month one of her planes 
assisted while the USS Buckley, one of her escorts, 
finished off a German U-boat after a short (16 min- 
utes) but thrilling surface engagement involving gun- 
fire and ramming. On May 29, while the task group 
was searching for a U-boat suspected to be in the 
vicinity, the Block Island was struck by three torpe- 
does in a short interval. One of the escorts had her 
stern blown off. Shortly thereafter, the other escorts 
made contact with the U-boat and probably sank her 
after two Hedgehog attacks. The gallant career of the 
Block Island came to an end later in the day when 
she had to be abandoned. 


Despite the enemy’s attempt to conserve his 
U-boats for the imminent invasion, the number of 
U-boats sunk during May 1944 was 29, the highest 
figure since the record total of 46 in July 1943. The 
leading area in the sinking of U-boats during May 
was the Northern Transit Area, East, where ten 
U-boats were destroyed during the month, nine of 
them by aircraft. Three of these were sunk by air- 
craft from HMS Fencer which was escorting a con- 
voy from Russia in the early days of the month. Dur- 
ing the latter part of May, a considerable increase 
was noted in the number of Baltic U-boats en route 
to the Atlantic via the Iceland-Faroes passage. For 
some time past the operations of Allied submarines 
against enemy shipping in Norwegian waters had 
prevented aircraft from making sweeps in this area. 
The enemy may have thought that he could safely 
relax his precautions in this area as many of these 
U-boats were traveling on the surface. Coastal Com- 
mand aircraft quickly exploited this soft spot, sink- 
ing six U-boats during the latter part of the month. 

The small number of U-boats in the Mediterra- 
nean was further depleted as four U-boats were de- 
stroyed there in May while another was sunk while 
attempting to pass through the Straits of Gibraltar. 
This was the third successful attack on a U-boat as a 
result of an initial MAD contact. The four successful 
attacks in the Mediterranean demonstrated the im- 
portance of persistence and the value of close coop- 
eration between air and surface units. Three of the 
four hunts involved aircraft as well as surface craft 
and the durations of the actions ranged from 22 
hours to 76 hours. This record action, which began 
on May 14, was culminated on the 17th with the 
sinking of a U-boat after a continuous hunt of 76 
hours involving eight ships and three planes. The 
U-boats in the Mediterranean fought back strenu- 
ously, sinking one merchant vessel and two escorts 
and damaging four other ships during the month. 

The outstanding event of the month, and probably 
the outstanding achievement of the U-boat war by a 
single ship, was the performance of the destroyer 
escort, USS England, in the Pacific from May 19, 1944 
to May 31. During this brief period, the England 
destroyed five Japanese U-boats and was assigned the 
major credit in the destruction of a sixth U-boat. 
The Hedgehog was the primary weapon used in 
achieving these results. This series of attacks resulted 
from the suspicion that a force of about five Japanese 
U-boats was patrolling the line of the Equator, to the 


COUNTERMEASURES TO THE U-BOAT 


53 


northeast of the Admiralty Islands. The USS Eng- 
land was accompanied by USS George and Raby 
when they swept through this area. The outstanding 
performance of the England is the more remarkable 
in that it was her first contact with the enemy. 

There was some credible evidence from aircraft 
sightings and attacks in the final week of May 1944 
that two or more U-boats were at sea in the Western 
English Channel, off the French coast. This proved 
to be a preview of the nature of U-boat operations in 
the next period as, for the first time since the early 
days of the war, the U-boats returned to the hazard- 
ous shallow coastal waters in the vicinity of England. 
This operation was possible only because the U-boats 
could take advantage of the use of Schnorchel and 
thereby reduce their exposure to aircraft attacks. 

62 COUNTERMEASURES TO THE 

U-BOAT 

Convoys 

During this period, the U-boats tried several modi- 
fications of their previous wolf-pack tactics in an 
effort to gain the upper hand in their attacks on 
Allied convoys. 

The first modification in tactics was made in Sep- 
tember 1943 in the attacks on the North Atlantic 
convoys. It was based on the use of the acoustic tor- 
pedo and involved attacking the escorts first, with 
the objective of reducing the convoy defenses to a 
point where the merchant vessels would become easy 
prey for the U-boats. Although some escort vessels 
were sunk, the objective was never accomplished; 
because at this stage of the war the Allies were using 
a larger number of escorts with the convoys and had 
a sufficiently large number of antisubmarine ships 
available that the loss of a few escorts would not 
seriously handicap future convoys. These U-boat 
tactics might have proved more effective in the early 
days of the war when the number of antisubmarine 
ships available to the Allies w'as extremely limited. 

The second modification was made in November 
1943 in tlie attacks on the convoys between Gibraltar 
and the United Kingdom. This change in tactics was 
forced on the U-boats by the heavy Allied air cover- 
age of the North Atlantic which prevented the 
U-boats from operating on the surface in the day- 
time and thereby prevented the concentration of a 
large number of U-boats around a convoy. The tac- 
tics adopted by the enemy involved stationing small 


packs of U-boats along the path of the convoy. Long- 
range aircraft from Bordeaux were used to shadow 
Allied convoys during the day. Reports of the air 
reconnaissance were passed on to the U-boat packs, 
which then attempted to maneuver into a favorable 
position for a night attack. This change in tactics 
came a little late, too, since flying facilities had be- 
come available in the Azores in October 1943. Several 
convoys were intercepted by the U-boats, but strong 
support by escort groups and land-based aircraft 
brought the convoys safely through with only neg- 
ligible damage. 

The third modification in U-boat tactics, made 
after the setback in November 1943, involved the 
almost complete abandonment of the old, highly 
organized wolf-pack attacks. Concentrations of 
U-boats were still maintained in the North Atlantic 
but attacks were generally made by individual 
U-boats who happened to find themselves in a favor- 
able position to attack a convoy. Although these 
U-boat tactics were much less effective against Allied 
shipping, they did enable the U-boats to remain sub- 
merged during the daytime. The scarcity of supply 
U-boats may have also been a factor in this drastic 
modification of wolf-pack tactics, which had been 
predicated on high-speed surface operations of the 
U-boats, requiring high fuel consumption. 

These futile attempts by the U-boats had very 
little success. Only about six ships a month were 
sunk by U-boats from Allied convoys. The tonnage 
sunk from convoys by the U-boats was only about 40 
per cent of the total tonnage sunk by them. The de- 
gree of safety reached by convoyed shipping during . 
this period is well illustrated by the experiences of 
the North Atlantic trade convoys (HX, SC, ON, 
ONS). Of the 900 ships that sailed monthly in these 
convoys, only about II /2 were sunk each month by 
U-boats. This represented a loss rate of only about 
one sinking per 600 transatlantic trips. This high 
degree of safety from U-boats was typical of the other 
convoy systems as well. 

One of the primary reasons for the ineffectiveness 
of the U-boats against Allied convoys was poor 
U-boat morale, as evidenced by their failure to press 
home attacks on convoys once they were detected. It 
is not difficult to see the reasons for this lack of ag- 
gressiveness. In the early months of 1943, Allied 
bombers had reduced the Biscay ports to heaps of 
stones. Though they could not get at the U-boats in 
their shelters, these raids deprived the U-boat crews 


54 


AIRCRAFT DEFEAT U-BOATS’ ATTEMPTED COME BACK 


of all but the most primitive facilities after they had 
come back, through continually mined waters, from 
long, exhaustive, dangerous, and now unsuccessful 
patrols. To reach the North Atlantic, the U-boats 
had to proceed submerged through the Bay of Biscay 
for the first five or six days of their patrol, surfacing 
for only a few hours after midnight. To find the 
convoy, the U-boats had to pass through the areas 
remorselessly swept by covering aircraft, and when 
the U-boats sighted the convoys, they found more 
numerous and better equipped escorts manned by 
more highly trained crews. 

An analysis of attacks on shipping by U-boats, in- 
dicating some of the factors governing the safety of 
ships against U-boat attacks, was made during this 
period and had considerable influence on Allied 
measures for the protection of shipping. This study 
indicated that the safety of independent ships de- 
pended very much on the speed of the ship, with the 
12-to-14 knot region being critical. A 12-knot ship 
had about three times the probability of being sunk 
as a 14-knot ship making the same trip. The explana- 
tion of this appears to be that the U-boat cannot, in 
general, follow a ship of 14 knots or above for any 
length of time and if it is not in a suitable position 
to make a submerged attack immediately, the ship 
will escape. The U-boat can follow slower ships on 
the surface, at a suitable distance, working round to a 
position from which it can attack on the surface at 
night. 

Speed was a significant factor in the safety of con- 
voyed shipping only when air escort was present. 
This study showed that the 9-knot convoys were con- 
siderably safer than the 7-knot convoys when air 
cover was available. When there was no air escort, the 
U-boat could proceed towards the convoy at high 
surface speeds and the extra 2 knots did little to pre- 
vent the U-boat from overhauling it. Another strik- 
ing result which emerged from this analysis was that 
the number of ships sunk was roughly independent 
of the size of the convoy as long as the number of 
escorts was the same. This meant that large convoys 
would lose, on the average, a smaller percentage of 
ships to U-boat attack and they would also be more 
economical in the use of escorts. 

The average size of the North Atlantic trade con- 
voys increased gradually during this period, rising 
from about 46 in March 1943 to about 57 in March 
1944. In April 1944 certain changes were made in the 
North Atlantic convoy schedules in order to allow 


several escort groups to be withdrawn from convoy 
duty for use in the invasion. The HX and ON con- 
voy speeds were changed from 10 knots to 8, 9, and 10 
knots in rotation, with the suffix S, N, and F desig- 
nating these respective speeds, while the slow SC 
and ONS convoys were abolished. The average sail- 
ing interval between convoys stayed at 7 i /2 days so 
that only four convoys sailed each way monthly, 
instead of six. This produced a substantial increase 
in the average size of these convoys. The average size 
of these convoys was 98 in May 1944 and HXM 292, 
consisting of 135 ships and six escorts, was the largest 
convoy of the war to that date. 

The danger from U-boat attack in the Western 
Atlantic was so low during most of this period that 
it was possible to allow some shipping from the U. S. 
coastal convoys to sail independently whenever a lull 
in U-boat activity occurred. The program was kept 
flexible in order to maintain a balance between the 
increased safety of convoyed shipping and the loss of 
time due to ships waiting in ports for convoys and 
sailing at slower convoy speeds. Fast merchant vessels 
(generally 1 1 knots and over), with the exception of 
those carrying high priority cargo, were able to sail 
independently along the coast during much of this 
period. Whenever the danger of a U-boat attack ap- 
peared imminent, independent shipping was ordered 
into port to await the next convoy. In some cases, 
where safety permitted, entire convoys were dis- 
persed and the ships proceeded alone to their 
destination. 

Aircraft 

The battle between U-boats and aircraft reached 
its climax at the beginning of this period, in July 
1943. The U-boats, equipped with improved anti- 
aircraft armament in the form of the quadruple 20- 
mm gun, were traveling on the surface during the 
daytime and were fighting back against aircraft. 
They also traveled through the Bay of Biscay in 
groups for mutual protection. 

As a result of their disastrous experience in July 
1943 when 34 U-boats were sunk in the Atlantic by 
aircraft, the U-boats were forced to abandon these 
aggressive tactics. The enemy realized that the 
U-boats were beaten and decided upon more cau- 
tious tactics that favored self-preservation of the 
U-boat fleet until new technical equipment and 
modified U-boats could turn the tide. These new 


COUNTERMEASURES TO THE U-BOAT 


55 


tactics involved complete submergence of the U-boats 
during the daytime. The fact that aircraft had swept 
the U-boats from the surface during the daylight 
hours constituted a real triumph for the Allies, as 
U-boats, robbed of their aggressiveness and mobility, 
lost a great deal of their effectiveness. 

The new tactics first became apparent in August 
1943 in the critical Bay of Biscay transit area. The 
U-boats reverted to surfacing at night for the mini- 
mum time necessary for charging batteries. In addi- 
tion, they made the transit hugging the coast of 
Spain, where it was rather difficult for the Leigh- 
Light Wellingtons to patrol because of their limited 
range. 

These changes had a profound effect on the Bay of 
Biscay campaign, and the productivity of Allied 
flying was greatly reduced. About 7000 hours were 
flown monthly in the Bay during the 5-month period 
from August 1943 through December 1943. These 
flying hours yielded only 12 sightings, six attacks, and 
one kill a month. Thus, although the flying effort was 
greater than during the preceding peak period (Feb- 
ruary 1943 - July 1943), the results achieved were 
only one-fifth as great. Only part of this decrease can 
be explained by the drop in the number of U-boat 
transits from 100 a month in the previous period to 
only about 45 a month during this period. The aver- 
age number of sightings per 1000 hours on patrol 
dropped to only two and only 25 per cent of the 
U-boat transits were sighted. The efficiency index 
(sightings per 1000 flying hours on patrol per 100 
U-boat transits) dropped from nine in the previous 
period to only four. 

The U-boats’ policy of maximum submergence 
during the daytime put great pressure on the devel- 
opment of suitable equipment and tactics to enable 
the air offensive to be maintained during the night. 
Considerable effort was devoted to the development 
of searchlights and flares that would improve the low 
effectiveness of night attacks, and the first attack on a 
U-boat by a U. S. searchlight-equipped plane was 
made in December 1943 by a PBM from Trinidad. In 
addition, squadrons were trained in night opera- 
tions from escort carriers. 

Coastal Command made a great effort to increase 
the amount of effective night flying in the Bay of 
Biscay by increasing the number of long-range 
searchlight-equipped planes. Leigh lights were fitted 
to Liberators as well as to additional squadrons of 
Wellingtons. The effectiveness of this increased 


amount of night flying was counteracted by the 
U-boats’ use of the Naxos GSR, which detected 
Allied S-band (10-cm) radar. The enemy started fit- 
ting his U-boats with this equipment in October 1943. 
The net effect of these changes was a small increase 
m the number of sightings in the Bay of Biscay. 
About 20 sightings and 15 attacks were made 
monthly in the Bay during the first five months of 
1944. However, the effectiveness of Allied flying in 
the previous peak period was not even approached. 

U. S. Navy planes participated in the Bay of Biscay 
offensive during this period under the operational 
control of Coastal Command. When the U-boat situ- 
ation along the East Coast of the United States 
started easing up in August 1943, the U. S. Army 
Anti-Submarine Command was withdrawn from 
antisubmarine operations. The U. S. Army Air 
Forces squadrons which had been operating in Eng- 
land were relieved by U. S. Navy Liberators. The 
U. S. Navy planes in England constituted Fleet Air 
Wing 7. 

Carrier-based aircraft emerged as one of the most 
dangerous enemies of the U-boat during this period. 
The U. S. escort carriers took the offensive against 
the U-boat in regions outside the range of shore- 
based aircraft, especially in possible refueling areas 
for the U-boats. This offensive was remarkably suc- 
cessful and rendered most ofthe Atlantic unsafe for 
surfaced U-boats. 

The U. S. escort carrier [CVE] operating with a 
screen of about four destroyers or destroyer escorts, 
composed a task group, assigned to antisubmarine 
warfare in the Atlantic. The primary mission of the 
task group was to protect convoys while the second- 
ary mission was to search out and destroy the U-boats. 
These task groups operated with the convoys be- 
tween the United States and Gibraltar. That the 
primary mission was accomplished may be seen from 
the fact that 2200 ships crossed the Atlantic in the 
GU and UG convoys from May 1943 through De- 
cember 1943 and only one ship was sunk by U-boat 
action. 

A study of attacks made by U. S. CVE-based air- 
craft during this period provides some evidence that 
their secondary mission was also fulfilled. About 28 
days were spent in U-boat waters on the average 
cruise and about 50 plane hours were flown per day. 
While in U-boat waters, one sighting was made for 
every 600 flying hours. Sixty of the 68 sightings 
studied resulted in an attack and about 40 per cent 


56 


AIRCRAFT DEFEAT U-BOATS’ ATTEMPTED COME-BACK 


of the attacks resulted in the sinking of the U-boat. 
The high quality of these attacks was due in a large 
part to the ability of the fast CVE-based planes to 
surprise the U-boat and attack it while it was still on 
the surface. Another factor in the success of these 
attacks was the cooperation between the F4F fighter 
planes, who strafed the U-boat, and the TBF 
bombers. 

The British escort carriers, operating in the very 
bad weather of the Arctic, played a major role in get- 
ting the Russian convoys through. The North Rus- 
sian convoys were started again about November 
1943 after having stopped running in March 1943. 
The sinking of the Scharnhorst was the most pub- 
licized part of this battle. The U-boats accomplished 
much less against these convoys during this period 
than in the past. During the period from December 
1943 through May 1944, the U-boats sank only five 
merchant vessels and two destroyers from these con- 
voys while 11 U-boats were sunk. British escort car- 
riers played a major role, contributing six of the 1 1 
U-boat kills. In one round trip of 16 days, with only 
eight days of operational flying, Swordfish planes 
from HMS Chaser sighted 21 U-boats, attacked 15, 
and with the aid of surface craft sank three of them. 

Rocket projectiles were used very successfully by 
the British carrier-based aircraft during this period. 
The first rocket attacks* by U. S. planes were made in 
January 1944 by aircraft from the USS Block Island, 
The U-boat was sunk; however, it was difficult to 
decide whether the sinking was due to the rockets 
or to depth charges which were also used in the 
attack. 

The lethality of aircraft attacks during this period 
was more than twice as high as it had been in the 
previous period. About 25 per cent of the aircraft 
attacks made on the U-boats in the Atlantic and 
Mediterranean resulted in the destruction of the 
U-boat while about 40 per cent of the attacks re- 
sulted in at least some damage to it. The effectiveness 
of aircraft attacks in the first half of 1944 was lower 
than the peak reached during the last half of 1943 
when some U-boats were still staying on the surface 
and fighting back. This falling off in effectiveness 
also reflected the U-boats’ tactics of maximum sub- 
mergence during the daytime as a much larger pro- 
portion of aircraft attacks, during the first half of 
1944, were made at night. The accuracy of night 
attacks was always much lower than that of day 
attacks. 


Scientific and Technical 

The decisive defeat suffered by the U-boats during 
the previous period seemed to have stimulated Ger- 
man scientific and technical progress and a number 
of radical changes were made in U-boat weapons and 
equipment during this period. 

One of the changes in U-boat weapons involved 
the replacement of the quadruple 20-mm antiaircraft 
gun mount by a new rapid-firing 37-mm antiaircraft 
gun. U-boat torpedoes were equipped with FAT gear 
for use against convoys. This gear caused the torpedo 
to describe a zigzag course and thereby increased the 
probability of a hit. The major change in U-boat 
weapons was the introduction of the T-5 torpedo, a 
21 -inch electrically driven acoustic homing torpedo. 
The speed of the T-5 torpedo was about 25 knots and 
its range about 6000 yards. This torpedo homed on 
the noise of the target’s propellers and its homing 
radius on a 15-knot escort was usually over 500 yards. 

The Allies immediately introduced countermeas- 
ures to the acoustic torpedo. The British twin 
FOXER (towed parallel-bar noisemakers used to 
decoy the acoustic torpedo from the ship) was born 
three months before the first acoustic torpedo attack 
took place. A complete escort group was fitted with 
twin FOXERS 17 days after the first attack. U. S. 
ships were fitted with FXR gear, which was similar to 
the British equipment but involved only a single 
noisemaker. Ships towing noisemakers or proceeding 
at speeds greater than 24 knots or less than 7 knots 
were considered to be relatively safe from acoustic 
torpedo attacks. Other ships were instructed to use 
the step-aside procedure when conducting attacks on 
U-boats which might fire an acoustic torpedo. This 
was a special tactical maneuver, involving a radical 
change in course when the U-boat is approached, de- 
signed to remove the escort vessel from the most prob- 
able danger area from acoustic torpedoes. 

The use of these towed noisemakers was unpopu- 
lar on many ships due to the inconvenience involved 
in streaming and recovering the gear, to the reduc- 
tion in maneuverability, and also to the interference 
with the sonar equipment. However, the noisemakers 
were undoubtedly effective against the acoustic tor- 
pedo. About 32 escorts and 19 merchant vessels are 
estimated to have been hit by acoustic torpedoes dur- 
ing the war. Very few of these casualties occurred 
when either the recommended tactical procedure 
was used, or the towed noisemakers were working 




COUNTERMEASURES TO THE U-BOAT 


57 


properly. Although the acoustic torpedo was more 
likely to score a hit than a normal torpedo, there 
were many cases of malfunctioning of the delicate 
mechanisms involved and those torpedoes that did 
hit were less likely to sink the ship due to the fact 
that they usually hit in the stern. 

The radar battle continued to play a prominent 
part in the U-boat war during this period. In the 
summer of 1943, Admiral Doenitz said, “The meth- 
ods in radio location which the Allies have intro- 
duced have conquered the U-boat menace.” How- 
ever, the German High Command did introduce 
some effective countermeasures in the fields of radio 
communication and radar detection during this 
period, mainly because German Intelligence finally 
became aware of the nature of Allied equipment. 

The major German scientific effort was put into 
the development of a search receiver that could de- 
tect Allied radar transmissions. At the beginning of 
this period the enemy still had no idea as to what had 
caused the huge increase in sightings and attacks on 
U-boats. The German search receiver had been im- 
proved by the fitting of a drum-shaped aerial (wire- 
basket) which did not have to be removed when the 
U-boat dived. A better receiver (Wanz G-1) had been 
perfected and was being fitted on U-boats. This 
served to reduce the amount of radiation from the 
set itself but did not help the U-boat situation at all 
as it could only detect radiation of wavelengths 
greater than 120 cm. The Germans then introduced 
the Borkum receiver, a much less sensitive crystal de- 
tector covering the 75- to 300-cm band, which pro- 
duced no radiation at all. 

Serious as were the immediate effects of these errors 
of judgment for the Germans, they extended far be- 
yond their original limits. They engendered in the 
minds of U-boat captains a fear lest they should be- 
tray their presence, and with this fear a distrust of 
Admiral Doenitz’ technical advisers. 

Finally, in September 1943, the U-boat command 
recognized that 10-cm radar was being used against 
them. The German Air Force had captured the blind 
bombing aid, H 2 S, which operated on the 10-cm 
band, in March 1943 and after a period of six months 
this information finally filtered through to the Ger- 
man Navy. As the simplest countermeasure, and still 
under the influence of their fear of radiating, the 
Germans produced in October 1943 a crystal detec- 
tor receiver, the Naxos, for the 8- to 12-cm band. The 
first models were crude portable units mounted on 


a stick. They were not pressure-tight and had to be 
passed below before diving. The maximum theo- 
retical ranges on Allied radar sets varied from about 
5 to 10 miles, but there was a considerable loss of 
efficiency under operational conditions. 

The introduction of the Naxos search receiver did 
reduce the number of sightings and attacks on 
U-boats and the number of disappearing contacts on 
Allied radar sets increased. However, many U-boats 
continued to be surprised successfully, due to the low 
efficiency of the gear, and it was evident to the Ger- 
mans that they still did not have the final solution to 
the radar problem. The U-boat command took the 
step of sending to sea a U-boat fully equipped to in- 
vestigate every type of Allied radar and carrying one 
of their best technicians with operational experience. 
He sailed from St. Nazaire in U-406 on February 5, 
1944, and was captured when the U-boat was sunk 
by HMS Spey on February 18. A similarly equipped 
U-boat suffered the same fate in April 1944. These 
losses probably set back the German radar counter- 
measures program considerably. 

Since the Allies were well aware of the potential 
effectiveness of an S-band search receiver, there were 
frequent false alarms reporting the introduction of a 
new GSR before any existed. The intelligence about 
Naxos, the increase in disappearing radar contacts, 
and the drop in U-boat sightings produced immedi- 
ate Allied reactions. There were a few cases where 
S-band radar sets were turned off. Steps were taken 
to develop attenuators, which caused a slow and 
steady decrease in transmitted power as the range 
closed, in order to confuse GSR operators. An in- 
terim tactic of a “tilt-beam” approach to reduce sig- 
nal intensity as the range closed was proposed but 
this proved rather difficult to carry out in actual 
practice. In addition, increased pressure was put on 
the development and fitting of X-band radar (3-cm 
wavelength). 

The general conclusion was that S-band radars 
should not be turned off, due to their much greater 
search width as compared to visual search and also 
to the fact that the Naxos sets were inefficient and 
sightings and attacks continued to be made on 
U-boats with GSR. In addition, the very fact that 
aircraft were causing the U-boats to submerge, there- 
by blinding and partially immobilizing them, greatly 
reduced the effectiveness of the U-boats in sinking 
ships. 

The Germans produced a number of other devel- 




58 


AIRCRAFT DEFEAT U-BOATS’ ATTEMPTED COME-BACK 



Figure 5. Attack by U. S. Navy Liberators (VPB-107) between the coast of Brazil and Ascension Island, November 5, 
1943. Note the radar antenna in its fairing on the left side of the conning tower, and just forward of it, an early form 
of the Naxos search -receiver antenna. The guns are still pointing at the previous position of the aircraft, while the 
gun crews seek cover from the 20-mm fire from the aircraft’s turrets. 


opments in the radar field. A radar decoy spar, de- 
signed to give false radar echoes was introduced, but 
proved ineffective. Some work was done on radar 
camouflage, with the object of developing rubber 
coatings for the U-boats that would deflect or absorb 
radar echoes. Earlier, there had been some attempt 
to use these rubber coatings against sonar, too. How- 
ever, the difficulties of producing rubber coatings 
that would work under operational conditions at sea 
were very great. The Germans also continued their 
experiments with infrared searchlights and receivers 
and took steps to prevent the detection of U-boats by 
imagined Allied infrared equipment. 

During this period, the U-boats finally began to 
appreciate the danger from Allied shipborne HF/DF 


as the result of a special Y party going to sea on a 
specially equipped U-boat. The U-boat command 
connected their diminishing success to some extent 
with their communications and tightened them up 
considerably. The lengths of messages were reduced 
and the frequencies used were changed more often. 
These shifts, while they made HF/DF both ashore 
and afloat more difficult, did not seriously reduce its 
use or effectiveness. They did make the task of 
U-boats attacking convoys more difficult. 

The most revolutionary technical development of 
this period was the fitting of Schnorchel to U-boats. 
The Schnorchel (or snout) is an extensible mast, con- 
sisting of an air induction trunk and a diesel exhaust 
line which are enclosed in a metal fairing. The cross 


COUNTERMEASURES TO THE U-BOAT 


59 



Figure 6. Close-up of U-1229, a 740-ton U-boat under attack by aircraft from USS Bogue, which later sank it. Note 
the extended Schnorchel just forward of the conning tower. 


section of this fairing is streamlined with the max- 
imum dimension about 20 inches. The mast is about 
26 feet in length and when raised is a few inches 
lower than the top of the extended periscope. The 
Schnorchel enables a U-boat to travel on its diesel 
engines at periscope depth at speeds of about 6 knots 
and also to charge batteries without surfacing. In 
essence, it was a defensive weapon, designed to re- 
duce the amount of time the U-boat had to spend on 
the surface and consequently to reduce the danger 
from air attack. 

The idea of an extensible air intake, which would 
enable a U-boat to charge its batteries while sub- 
merged, had been current in the German Navy in 
pre-war years but was first brought forcibly to its 


attention by the capture, in 1940, of two Dutch sub- 
marines fitted with such equipment. No steps were 
taken to follow up this idea while things were going 
well for the U-boats, and it was not developed until 
the end of 1943, after aircraft had gained the upper 
hand over U-boats. The first U-boats were not fitted 
with Schnorchel until February 1944, and it was not 
until June 1944, the start of the next phase of the 
U-boat war, that its effect on U-boat operations be- 
came significant. 

The U. S. Navy introduced during this period a 
number of new devices designed to improve the sonar 
performance of its ships. The Bearing Deviation 
Indicator [BDI] was one of the most helpful of these 
sonar aids. BDI was used with standard echo-rang- 


60 


AIRCRAFT DEFEAT U-BOATS’ ATTEMPTED COME-BACK 


ing equipment to let the operator see, for every ping, 
whether the target producing the echo was to the 
right or left of the bearing of the projector The 
operator could, therefore, determine the bearing of 
the target with greater accuracy and rapidity than 
with standard echo-ranging equipment alone. 

Two new types of depth charges came into use on 
U. S. ships during this period. The Mark 8 depth 
charge was fitted with a magnetic proximity fuze 
which was designed to detonate when within lethal 
range of the U-boat. The Mark 9 depth charge was 
steel-cased and shaped so as to have a fast sinking 
rate, thereby reducing the blind time and conse- 
quently the effect of U-boat maneuvers. Both these 
new depth charges were designed to increase the 
lethality of surface craft attacks on U-boats. 

The British also improved the antisubmarine 
equipment on their ships during this period, but 
along different lines than the U. S. Navy. The 
Squid, which is designed to throw depth charges 
ahead of the ship and is automatically controlled and 
operated, came into service at the beginning of 1944. 
It employs a 3-barreled mortar, electrically fired, de- 
signed to discharge bombs ahead of the attacking 
ship. These bombs closely resemble depth charges in 
weight and explosive effect but have the following 
advantages over depth charges: 

1. They are projected with accuracy to a known 
point, well ahead, while the attacking ship is still in 
Asdic contact with the U-boat. 

2. They have a reliable underwater course. 

3. They have a much higher sinking speed. 

4. They incorporate a new type of fuze which is 
set automatically to the required depth with a high 
degree of accuracy. 

The depth is set on the fuzes electrically from the 
new Type 147B Asdic depth predictor. The mortars 
are fired automatically from the Asdic range re- 
corder. The three charges are thrown to the points 
of an equilateral triangle. When two mortars are 
fitted, as in frigates, the pattern is in two depth layers. 

The Type 147B Asdic depth predictor was de- 
veloped primarily for use with the Squid, so as to 
obtain a measurement of the depth of the U-boat 
just before the projectiles are fired. However, the 
Type 174B depth predictor, in conjunction with the 
appropriate range and bearing recorders and Q at- 
tachment, can also be used in Hedgehog and depth- 
charge attacks. Type 147B uses a fan-shaped beam of 
high-frequency sound (50 kc) which may be de- 


pressed down to 45° below the horizontal. The beam 
of sound is broad in the horizontal plane (30° to 40° 
on either bow) and narrow in the vertical plane (2° 
to 3° off axis). Trials have shown that the set is cap- 
able of setting the Squid fuzes to within about 20 
feet of the depth of the U-boat, provided it is below 
100 feet. At shallower depths it is not possible to get 
accurate measurements. It is possible to make accu- 
rate attacks on U-boats at depths down to some 800 
feet, provided that the Q attachment is fitted. 

Sinkings of U-boats 

The average number of U-boats sunk or probably 
sunk monthly reached a peak of 21 a month during 
this period. The record monthly score of the war was 
reached in the first month, July 1943, when 46 
U-boats were destroyed throughout the world. Of 
the total of 234 U-boats destroyed during this ll 
month period, 199 were German, 28 were Japanese, 
and 7 were Italian. The 180 U-boat sinkings in the 
Atlantic were rather widely distributed with the 
leading areas being the Northwest Atlantic Area 
with 34 kills, the Northeast Atlantic Area with 29 
kills, and the Biscay Channel Area with 28 kills. 

Aircraft continued to be the leading U-boat killers 
during this period accounting for 122 alone (52 per 
cent of the world-wide total) and another 22 (9 per 
cent of total) with the cooperation of surface craft. 
Carrier-based aircraft accounted for 40 of the 144 
kills in which aircraft participated. Ships accounted 
for 79 U-boats (34 per cent of total), and Allied sub- 
marines accounted for the other 11 U-boats (5 per 
cent of total). 

The quality of surface craft attacks continued to 
improve steadily through this period as the crews 
became more experienced and improved weapons 
and equipment became available. About 30 per 
cent of the surface craft attacks made on U-boats in 
the Atlantic and Mediterranean during this period 
resulted in at least some damage to the U-boat while 
20 per cent of the attacks resulted in the sinking of 
the U-boat. The lethality of these attacks was twice 
as high as it had been during the previous period. A 
study of British assessed attacks during this period 
indicated that about three patterns were dropped in 
the average attack on a U-boat. The probability of 
sinking a U-boat was about 6 per cent for the average 
depth-charge pattern and about 1 1 per cent for the 
average Hedgehog pattern. 


SURVEY OF RESULTS 


61 


6 3 SURVEY OF RESULTS 

6 . 3.1 From the U-boats’ Point of View 

To interpret the results of the U-boat war during 
this period correctly, it is important to realize that 
the German High Command appreciated, at the be- 
ginning of this period, that the U-boats had been 
decisively defeated in the crucial battle against the 
North Atlantic convoys. The enemy also realized 
that the old type of U-boat would have to be modi- 
fied radically before it could again be a serious men- 
ace to the Allied supply lines across the Atlantic. 

The solution reached by the German High Com- 
mand was the development of an entirely new type 
of U-boat, more immune from radio location, with 
much greater submerged speed, and an entirely new 
and quicker method of production. The first sketch 
of what was to be the new prefabricated Type XXI 
U-boat was made in July 1943. By December 1943 all 
designs were finished, including full-scale wooden 
mockups. In February 1944 it was decided to stop 
the construction of the old-type U-boats, except that 
those that had already been laid down were to be 
completed, and to concentrate on the production of 
the new prefabricated U-boats. 

The general strategy of the enemy was to keep the 
U-boat fleet in being until the new U-boats would be 
ready, when they could return to the offensive again. 
In line with this policy, the Schnorchel was valuable 
as an interim measure, to reduce the danger from air 
attack and to enable the U-boats to operate in in- 
shore waters in the event of invasion. The main aim 
of the U-boats during this period was self-preserva- 
tion, although they would attempt in the meantime 
to sink as much Allied shipping as they could with- 
out suffering excessive losses. In addition, keeping 
the U-boats at sea served to tie up large Allied forces 
in the protection of shipping, thereby keeping them 
from being used against Germany in other ways. 

During the last half of 1943, the U-boats made 
several attempts at conducting offensive operations 
but each time they were beaten back with heavy 
losses. Thereafter, the U-boats seemed to realize that, 
if they were to accomplish their primary mission of 
self-preservation, they could not undertake any large- 
scale offensives against shipping. The number of 
U-boats at sea in the Atlantic dropped steadily, 
reaching a minimum of about 40 in May 1944. The 
main functions of these U-boats, during the first five 
months of 1944, were probably reconnaissance. 


weather reporting, forcing the Allies to convoy ship- 
ping, and waiting for the invasion. They made no 
attempt to sink a large amount of Allied shipping. 
This is clearly reflected in the results achieved by 
the U-boats during this period. 

World-wide shipping losses to U-boats reached a 
new low as only about 17 ships of 101,000 gross tons 
were sunk monthly by U-boats. This was only about 
one-fourth the amount sunk monthly by U-boats dur- 
ing the previous period. Only about 45 per cent of 
these sinkings took place in the Atlantic, as about 
40 per cent occurred in the Indian Ocean and an- 
other 15 per cent in the Mediterranean. The world- 
wide number of U-boats sunk monthly reached a 
peak of about 21 a month during this period. The 
world-wide exchange rate reached a new low of only 
^ of a ship (4800 gross tons) sunk by the average 
U-boat before it itself was sunk. 

U-boat activity in the Mediterranean was slightly 
lower than in the preceding period. Sicily was in- 
vaded in July 1943 and the Mediterranean was con- 
sidered open for Allied shipping, although almost 
all of it was forced to travel in convoy. Only 36 mer- 
chant vessels were sunk by U-boats during this period 
at a price of 23 U-boats sunk. 

Shipping losses to German and Japanese U-boats 
in the Indian Ocean were at a slightly higher level 
than previously as 71 ships were sunk during this 11 
month period. However, this was the first period in 
which there was some evidence of countermeasures 
against the U-boats in the sinking of seven U-boats. 
This was one of the few areas where the exchange 
rate was still favorable for U-boat operations as 10 
ships were sunk by U-boats for each U-boat sunk. 

Japanese U-boats were strictly on the defensive in 
the Pacific and spent most of their time in supplying 
isolated outposts. Despite the fact that the bulk of 
the shipping in the Pacific sailed independently, only 
one merchant vessel was sunk by U-boats during this 
11-month period, while Allied forces, mostly surface 
craft and submarines, sank 24 Japanese U-boats in 
the Pacific.® 

The average number of U-boats at sea in the At- 
lantic during this period was only 61, about 40 per 
cent less than the average number at sea during the 
previous period. These U-boats sank only eight ships 
of 44,000 gross tons per month in widely scattered 
areas of the Atlantic. The average U-boat in the At- 

a Four more Japanese U-boats were sunk in the Indian Ocean, 




62 


AIRCRAFT DEFEAT U-BOATS’ ATTEMPTED COME-BACK 


lantic reached a new low in offensive power as it was 
able to sink only i/^ of a ship (700 gross tons) per 
month at sea. 

Despite the meager results achieved by the average 
U-boat and the smaller number of U-boats at sea, the 
number of U-boats sunk in the Atlantic continued to 
increase as 16 were sunk monthly during this period. 
The average life of a U-boat at sea in the Atlantic 
during this period was, therefore, only about 4 
months, half the average life of a U-boat at sea in 
the previous period. This meant that the average 
U-boat at sea in the Atlantic was able to sink only I /2 
of a ship (2800 gross tons) before it itself was sunk. 
The magnitude of the disaster which the U-boats 
suffered during this period may be roughly measured 
by the fact that the exchange rate (ships sunk per 
U-boat sunk) was about nine times as high in the 
previous period, the battle against the North Atlan- 
tic convoys, and about 38 times as high in the peak 
period, the first 9 months of 1942. 

These figures reflected the growing strength and 
efficiency of Allied surface and air craft and the 
widening gap between Allied weapons and equip- 
ment and that of the U-boats. They also confirmed 
the German High Command’s appreciation, at the 
beginning of this period, that the old-type U-boat 
could not compete any longer with Allied antisub- 
marine forces. The only consolation the enemy 
could have, at the end of this period, was that the 
U-boat losses would probably have been much larger 
than they actually were if the U-boats had actually 
continued their large-scale offensive against Allied 
shipping. This is indicated by a comparison of the 
16 U-boats sunk monthly in the Atlantic with the 
37 U-boats sunk in July 1943 and the 25 U-boats 
sunk in October 1943, the months in which the 
U-boats attempted offensive operations. 

By operating their U-boats as they did, the Ger- 
mans were able to maintain their large U-boat fleet 
for the invasion. About 250 new German U-boats 
were constructed during this period while only about 
200 German U-boats were sunk so that the Germans 
had over 400 U-boats at the end of this period. 

6.3.2 From the Allies’ Point of View 

The Allied and neutral nations lost about 184,000 
gross tons of shipping monthly from all causes during 
this period, only about a third of the monthly ship- 
ping losses during the preceding period, while the 


construction of new merchant shipping ran at about 
1,160,000 gross tons monthly, slightly higher than 
during the preceding period. Consequently, there 
was a net gain of about 976,000 gross tons a month 
in the amount of shipping available. 

The shipping available to the Allies had increased 
by almost 11,000,000 gross tons during this period to 
a total of about 7500 ships of 47,500,000 gross tons. 
About 10,500,000 gross tons of this total consisted of 
tankers. October 1943 was the first month in which 
the total shipping available was larger than the 
40,000,000 gross tons of shipping available at the 
start of the war, in September 1939. ft took over four 
years to replace the heavy shipping losses caused 
mostly by U-boat action in the early years of the war. 

By the end of this period, the shipping crisis had 
definitely passed and the Allies had sufficient ship- 
ping available to undertake the invasion. In fact, 
construction of new merchant shipping had started 
tapering off in 1944 after reaching a peak of l,5OO,OO0 
gross tons in December 1943. Part of this tapering off 
was due to the conversion of shipyards to the con- 
struction of the faster Victory ships which took about 
twice as long to build as the slower Liberty ships. 

Of the 184,000 gross tons of shipping lost monthly 
from all causes, 147,000 gross tons were lost as a re- 
sult of enemy action. U-boats accounted for 101,000 
gross tons a month, about 69 per cent of the total 
lost as a result of enemy action. Monthly shipping 
losses due to enemy aircraft were higher than in the 
preceding period, amounting to 34,000 gross tons a 
month, about 23 per cent of the total due to enemy 
action. Shipping losses from enemy surface craft, 
mines, and other enemy action were even lower than 
in the preceding period, totaling only 22,000 gross 
tons a month, or 8 per cent of the total. 

The major task of the Allies during this period 
had been accomplished. Sufficient supplies and men 
had been landed in England during this period to 
enable the Allies to undertake the invasion of Europe 
in June 1944. Although the U-boats were no longer 
the serious menace they had been in the past, the 
large U-boat fleet based on both flanks of the English 
Channel constituted a potential threat to the success 
of the invasion. The immediate problem facing the 
Allies, at the end of this period, was to prevent the 
U-boats from attacking the large concentration of 
shipping that would be carrying troops and military 
equipment across the Channel during the invasion. 
From a long-term point of view, it was necessary to 


SURVEY OF RESULTS 


63 


maintain the numerical and technical superiority of ing the U-boats from ever cutting the large and con- 
Alhed antisubmarine forces over the U-boats. Allied tinuous flow of supplies that would be required bv 
antisubmarine forces faced the problem of prevent- Allied fighting forces in Europe. 


Chapter 7 

SEVENTH PERIOD 

SCHNORCHEL U-BOATS OPERATE IN BRITISH HOME WATERS 

JUNE 1944-END OF WAR 


7 1 U-BOAT OFFENSIVE 

I N MANY respects this last period of the U-boat war 
resembled the first period. The U-boat war was 
again subsidiary to, and largely influenced by, mili- 
tary operations in Europe. The U-boats were driven 
from their bases on the Bay of Biscay, which they had 
used for four years, and were forced to return to 
bases in Norway and the Baltic. The waters around 
the British Isles again became the main area of 
U-boat activity, as Schnorchel-equipped U-boats 
were able to operate in inshore waters with relative 
safety from aircraft detection. The U-boats were 
operating mostly submerged, not from choice as in 
the first period, but because they had been forced 
under by Allied aircraft. Surface craft again became 
the main destroyers of U-boats. 

The U-boats found that operations in inshore 
waters were just as hazardous during this period as 
they had been at the beginning of the war. The chief 
difference was that during this period the average 
U-boat was able to sink only one-twentieth the 
amount of shipping that it sank in the first period. 
Allied escorts were more numerous, more experi- 
enced, and better equipped, while the U-boats’ offen- 
sive power was greatly reduced by their loss of mo- 
bility. U-boat morale was also much lower during 
this period than it had been early in the war. 

The lull in U-boat activity ended with the Allied 
landings in Normandy on June 6, 1944. The Allies 
had appreciated that the U-boats were a very serious 
potential threat to the success of the invasion. The 
size of the U-boat fleet at that time was still such 
that a mass attack in the Channel on D-day, from 
both flanks, might have saturated the defenses and 
inflicted grave losses on Allied convoys during the 
critical early days of the operation. 

The Allied countermeasures involved blocking off 
the cross-Channel Area and guarding the convoy 
routes leading to it, with the object of making the 
approaches to both a difficult and exhausting opera- 
tion for the U-boats. By June 6, ten escort groups. 


consisting of 54 ships, were ready to block the west- 
ern approaches to the Channel. They were sup- 
ported by three escort carriers, which were there 
chiefly to provide fighter support to escorts operat- 
ing close to the enemy shores. The enemy air threat 
proved, however, to be so slight that these carriers 
were withdrawn by June 11. Coastal Command air- 
craft put on an intensive flying effort in the Channel 
and its western approaches and also in the Bay of 
Biscay Area. These dispositions of anti-U-boat forces 
were independent of, and in addition to, the escorts 
and aircraft provided for close escort duties with the 
convoys running to and from France. 

The first reaction of the U-boats to the invasion 
was a considerable exodus of U-boats from the 
French ports on D-day, as soon as the enemy woke up 
to the fact that the operation had really started. The 
number of U-boats in the Biscay-Channel Area in- 
creased from one on June 5 to about 20 on June 8. 
The majority of these U-boats made no attempt to 
enter the Channel but, instead, set up defensive 
patrols off the Biscay ports to counter possible inva- 
sion attempts in that area. 

A curious feature of these operations was that they 
began with a large number of sightings and attacks 
on U-boats by aircraft and then, after a period of 
several days, the number of contacts was sharply re- 
duced. This reduction was due to the extensive use of 
Schnorchel by the U-boats, which had begun at that 
time. It seems that the U-boat captains had not taken 
very kindly to the Schnorchel with all the discom- 
forts that it was capable of causing in inexperienced 
hands, and intended to use it only as a last resort, if 
the weight of air power against them became intol- 
erable. After six U-boats wTre sunk by aircraft attacks 
in the Biscay Area between June 7 and 10, the U- 
boats began to appreciate the value of Schnorchel 
and quickly learned how to use it efficiently. 

It is also possible that the enemy intended to op- 
erate his U-boats in the Channel, accepting all risks 
and proceeding on the surface in order to reach the 
vital invasion area and disrupt the Allied landing 


GgifFIBgg ^AL 


64 


U-BOAT OFFENSIVE 


65 


operations. It was the initial blows dealt him by 
Coastal Command in the early days of the operation 
that forced him to adopt the more cautious, and less 
effective, tactics of remaining continuously sub- 
merged, which the advent of Schnorchel gave him 
the means of doing. The Schnorchel was a very poor 
radar target, even at short range, and was difficult to 
sight, even in daylight. To attack it at night required 
exceptionally good radar tracking and Leigh-light or 
flare technique. 

In addition to their successes in the Bay of Biscay, 
Coastal Command aircraft contributed an outstand- 
ing performance in June against U-boats in Nor- 
wegian waters. Most of these U-boats were probably 
en route for the English Channel to join in opera- 
tions there. These U-boats continued to operate 
mainly on the surface, and Coastal Command air- 
craft sank seven in Norwegian waters during June. 

The first surface craft kill in the Biscay-Channel 
Area did not come until June 18. This was followed 
by four other surface craft kills, two with the help 
of aircraft, before the end of the month. There was 
only one aircraft kill in the Biscay-Channel Area be- 
tween June 1 1 and the end of the month. 

The U-boats obtained their first successes against 
merchant shipping in the invasion area more than 
three weeks after D-day, and sank five ships in the 
last few days of June. Two escort vessels were tor- 
pedoed by U-boats in the middle of June. When 
these results are measured against the 12 U-boats 
sunk in the same area in June by Allied forces, it is 
clear that the U-boats failed completely in their at- 
tempt to disrupt the Allied invasion. 

The other enemy weapons (aircraft, surface craft, 
and mines) were equally ineffective against the thou- 
sands of Allied ships that moved across the Channel 
to Normandy. Only 18 ships of 75,000 gross tons were 
sunk in the Biscay-Channel Area in June 1944, as a 
result of all forms of enemy action. The Allies scut- 
tled over 50 merchant vessels of about 300,000 gross 
tons in constructing the artificial ports that were 
used so successfully during the invasion. 

The enemy turned to other offensive weapons in 
July, such as the human torpedo, explosive motor 
boats, and the V-1 bomb, but they did very little 
damage to Allied shipping. The experience in the 
Biscay-Channel Area during July and August was 
similar to that in June. The Allies continued to 
keep up their pressure on the U-boats, while the 
U-boats found the approaches to the shipping lanes 


so difficult that those who reached them fumbled 
the opportunities they found. U-boats operating in 
this area sank only two merchant vessels in July and 
six in August, while the Allies sank nine U-boats in 
July and 12 in August. 

Surface craft played the predominant role in sink- 
ing U-boats in this area during these two months, 
killing 13 of them alone and destroying three others 
with the assistance of aircraft, out of the total of 21 
sunk. An outstanding feature of the surface craft 
attacks in the invasion area was the difficulty experi- 
enced in the initial detection of U-boats which 
adopted anti-Asdic tactics of resting on the bottom 
when escort vessels were heard approaching. Numer- 
ous wrecks, together with the difficult water condi- 
tions and the high reverberation background in 
shallow waters, gave the U-boat almost complete im- 
munity from Asdic detection. However, once the 
Asdic picked up the U-boat contact and identified it 
as such, the chance of obtaining a kill rose to a peak 
of about 50 per cent in the invasion operation. 

The Second Escort Group continued its brilliant 
career during a ten-day patrol in the Biscay-Channel 
Area early in August. It achieved the first destruc- 
tion of a U-boat by Squid and also took part in the 
sinking of two other U-boats. The Squid kill was re- 
markably quick. One pattern of six Squid charges 
was sufficient to bring the U-boat to the surface, and 
the crew rapidly abandoned the sinking U-boat. 

The breakthrough of Allied land forces across the 
Cherbourg peninsula in the first week of August 
threatened the enemy’s Biscay ports and forced their 
evacuation. The U-boats abandoned their efforts on 
the Biscay-Channel Area toward the end of August 
and headed for Norwegian ports. This marked the 
end of the first phase of the U-boat campaign against 
cross-Channel Allied shipping. 

The enemy concentrated his main effort against 
the Allied invasion during the first three months of 
this period, and U-boat activity in other areas was 
slight. The average number of U-boats at sea in the 
Atlantic during June 1944 was about 48, and over 
half of these were concentrated in the Biscay-Chan- 
nel Area and the Northern Transit Area-East. 
World-Wide shipping losses to U-boats continued to 
be low as only 1 1 ships of 58,000 gross tons were sunk 
by U-boats in June. Besides the five ships sunk in the 
Biscay-Channel Area, there were three ships sunk in 
the rest of the Atlantic and three ships sunk in the 
Indian Ocean. 


66 


SCHNORCHEL U-BOATS OPERATE IN BRITISH HOME WATERS 



Figure 1. Boarding party from USS Guadalcanal labors to keep the captured U-505 afloat after its crew had aban- 
doned it to sink. 


The total number of U-boats sunk during June 
was 28, about the same as the high level reached in 
May. Besides the 21 U-boats sunk in the waters 
around the British Isles, four others were disposed of 
in widely distant parts of the Atlantic by U. S. escort 
carrier groups. One of these four was U-505, which 
was captured by USS Guadalcanal [CVE] and her 
escorts on June 4, 1944. 

This task group sailed from Norfolk in May 1944 
with the avowed intention of capturing an enemy 
submarine. It was felt that there was a good oppor- 
tunity to capture a U-boat that surfaced, by concen- 
trating anti-personnel weapons on it, holding back 
on weapons that would sink it, and making an at- 
tempt to board it as soon as possible. After an unpro- 
ductive hunt around the Cape Verde Islands, a well- 
conducted search plan was put in operation on May 
31 for an estimated homebound U-boat. The USS 
Chatelain [DE], one of the escorts, made sonar con- 
tact at about 1000 on June 4. A Hedgehog attack, 
followed by a shallow depth-charge attack, brought 


the U-boat to the surface at 1023 and fire was opened. 
The U-boat crew scrambled on deck and dived over- 
board. At 1027, “Cease firing” was ordered. 

The U-boat was then running in a tight circle at 
about seven knots, fully surfaced, and it was known 
that most of her crew had abandoned her. USS Pills- 
bury [DE], another escort, lowered a whaleboat with 
a boarding party and then attempted to rope the 
U-boat. Meanwhile, the boarding party got along- 
side and leaped from the whaleboat to the deck of 
the circling U-boat. There was only one dead man 
on deck and no one below, and the boarding parties 
immediately set to work closing valves and discon- 
necting demolition charges. The Guadalcanal took 
the U-boat in tow, but many difficulties were en- 
countered as the towlines broke and the U-boat 
showed signs of settling. It was not until June 8 that 
the U-boat was at fully surfaced trim. This was the 
first time that a U-boat had been captured by the 
U. S. forces. 

U-505 was finally brought to Bermuda and the 




U-BOAT OFFENSIVE 



Figure 2. A torpedo plane approaches for a landing while USS Guadalcanal tows U-505 astern. 


Allies were able to extract a great deal of extremely 
valuable technical information from the manuals 
and equipment aboard the U-boat. In addition to 
more reliable data on the acoustic torpedo, German 
search receivers, and other standard U-boat equip- 
ment, the Allies obtained important information 
about German war orders, communications, and 
codes. Much of this information proved to be of 
value in conducting later operations against the 
U-boats, as the Germans did not know that we had 
captured a U-boat and obtained this information. 

AV^orld-wide shipping losses stayed low in July 
1944 as only 12 ships of 63,000 gross tons were sunk 
by U-boats. In addition to the two ships sunk in the 
Biscay-Channel Area, five ships were sunk in the rest 
of the Atlantic and five in the Indian Ocean. The 
number of U-boats sunk throughout the world in 
July was 22, slightly less than in June. Escorts of 


USS Card [CVE] eliminated a potential threat to 
Allied shipping by sinking a 1600-ton minelaying 
U-boat early in July. This U-boat carried 66 mines, 
the moored acoustic type, which were intended to be 
laid in the approaches to Halifax. 

August 1944 was the best month of this last period 
for the U-boats, as they sank 18 ships of 99,000 gross 
tons throughout the world. This peak score, how- 
ever, is only about the same as the average monthly 
sinkage achieved by U-boats during the previous 
period, which was a rather low average, at that. Be- 
sides the six ships sunk in the Biscay-Channel Area, 
there were only two ships sunk in the remainder of 
the Atlantic. U-boats sank one ship in the Black Sea 
and nine ships in the Indian Ocean, mostly off East 
Africa in the Mozambique Channel. The landings in 
Southern France took place during August without 
the loss of any ships in the entire invasion area. 


68 


SCHNORCHEL U-BOATS OPERATE IN BRITISH HOME WATERS 


The downward trend in the number of U-boats 
sunk monthly continued during August as only 17 
were sunk. Aircraft found it more difficult to detect 
and attack the Schnorchel U-boat. Apart from the 12 
U-boats sunk in the Biscay-Channel Area, there were 
only three U-boats sunk in the Atlantic, all by escort 
carrier task groups. Aircraft from HMS V index, 
which was escorting the North Russian convoys, sank 
two U-boats, while aircraft from the USS Bogue sank 
U-1229 south of Newfoundland, after a 20-day 
search. This U-boat, traveling on Schnorchel, h?d 
remained submerged for about 14 days, surfacing 
only for a short daily interval of 10 to 15 minutes to 
determine her position. 

The most significant development in August, with 
regard to future U-boat operations, was the advance 
of Allied armies in W^estern France which forced the 
U-boats to abandon their bases on the Bay of Biscay. 
These bases contained the redoubtable Todt con- 
crete shelters which had withstood heavy Allied at- 
tacks for years. The basic strategy of German naval 
warfare in the Atlantic had depended upon the suc- 
cessful use and maintenance of the Biscay bases, 
from which U-boats could proceed directly to their 
operational areas with minimum fuel consumption 
and less throttling opposition from Allied air and 
surface patrols. 

By the end of August it was evident that the 
U-boats had begun their final exodus from the Biscay 
ports and were heading for Norway, where they 
would be much more vulnerable to Allied air attack. 
The problems of repair and maintenance of a large 
U-boat fleet at the small and inadequately equipped 
Norwegian bases would also be much more difficult. 
The main effect, however, of the loss of the Biscay 
bases was the considerable increase in the length of 
voyages to operating areas. The 500-ton U-boats, 
which comprised the majority of the U-boat fleet, 
were thereafter restricted to operations around the 
British Isles, as it was extremely difficult to refuel at 
sea. Even the 740-ton U-boats confined most of their 
later operations to the nearby Atlantic, operating 
near Canada and Gibraltar. 

During September 1944 the enemy seemed to be 
concerned primarily with shifting his U-boats from 
the Bay of Biscay to Norwegian bases. About 25 
U-boats were engaged solely in running the gauntlet 
of Allied air and surface patrols between France and 
Norway. These U-boats traveled submerged for al- 
most the entire trip, using Schnorchel, and most of 


them completed the journey safely. As a result of this 
mass transit of U-boats, the average number at sea in 
the Atlantic reached a peak for this last period dur- 
ing September when there wqre 57 U-boats at sea. 

World-wide shipping losses were considerably re- 
duced in September as only seven ships of 43,000 
gross tons were sunk by U-boats. A noteworthy fea- 
ture was the almost complete absence of U-boat ac- 
tivity in the Indian Ocean. One ship was sunk there 
early in the month, but the U-boat responsible for 
this sinking withdrew towards Penang where it was 
torpedoed by a British submarine. Although the 
other six ships sunk by U-boats during the month 
were all lost in the Atlantic, not a single ship was 
sunk in the formerly active Biscay-Channel Area. 
Two ships were sunk in the Barents Sea Area and 
one in the Canadian Coastal Zone. The other three 
ships were sunk early in September, from North At- 
lantic convoys, by U-boats operating on Schnorchel 
in inshore waters. These U-boats operated in the 
Northwestern Approaches to England, in an area of 
high shipping density through which the North At- 
lantic convoys passed on their way in and out of the 
North Channel. Fortunately, no further losses oc- 
curred in this area, as the Allies started routing the 
North Atlantic convoys around the south of Ireland 
at about this time. 

The total number of U-boats sunk in September 
was 21. This included, however, six 300-ton U-boats 
that were scuttled in the Black Sea as a result of Rus- 
sian advances in eastern Europe. In the Mediterra- 
nean, two of the three U-boats based at Salamis were 
destroyed. The general clearance of U-boats and 
enemy aircraft from the Mediterranean enabled the 
number of ships employed in convoy escort to be re- 
duced and a large number of independent sailings 
was permitted. Only ten U-boats were sunk in the 
Atlantic. This steady decrease reflected the inactivity 
of the U-boats and the increased effectiveness of 
Schnorchel. Two of these U-boats were sunk as a 
result of the inshore activity in the Northwestern 
Approaches to England, five were sunk in the North- 
ern Transit Area-East, and three in the remainder 
of the Atlantic. 

By the end of September the last of the U-boats 
appeared to have departed from the Biscay-Channel 
Area. The average number of U-boats out at sea in 
the Atlantic declined to about 30 in October, with 
most of them still in transit. The results achieved by 
German U-boats reflect this situation, as October 


U BOAT OFFENSIVE 


69 


1944 was the first month of the war during which 
they were not able to sink even a single ship in the 
Atlantic. As a matter of fact, the U-boats sank only 
one ship of 7000 gross tons during October. A perfect 
record for the month was spoiled on October 30, 
when a Japanese U-boat sank a U. S. cargo ship' 
traveling independently in the Pacific, midway be- 
tween San Francisco and Pearl Harbor. Ironically, 
this was the first ship sunk in the Pacific by a U-boat 
since November 1943. The lull in U-boat activity 
was reflected on the east coast of the United States, 
as October was the first complete month of inde- 
pendent sailings for all ships engaged in coastal 
trade (except for dry cargo vessels of 8 to 10 knots). 

Only nine U-boats were sunk during October, five 
of them in the Pacific and only four in the Atlantic. 
All four of the latter were sunk in the Northern 
Transit Area— East, the area through which the 
U-boats traveled to and from their Norwegian bases. 
One of these four U-boats was sunk as the result of a 
bombing raid on Bergen. One of the five U-boats 
sunk in the Pacific was a German U-boat, sunk by 
the Dutch submarine HNMS Zwaardvisch in the 
Java Sea. This was by far the most easterly position 
in which a German U-boat had ever been destroyed. 

Although the average number of U-boats at sea 
in the Atlantic during November was only 24, the 
lowest monthly average of this period, the enemy had 
completed his transfer of U-boats from the Biscay 
bases, and the U-boats at sea were more active. Seven 
ships of 30,000 gross tons were sunk by U-boats in 
November 1944; two ships in the Indian Ocean, a 
Swedish ship in the Baltic, and four ships in inshore 
waters in the Atlantic. Three of these ships were sunk 
on November 10 in the approaches to Rekjavik, Ice- 
land. The other ship was sunk in the English Chan- 
nel toward the end of the month after several months 
without any U-boat activity in this area. 

The number of U-boats sunk during November 
was 10, but seven of these U-boat kills took place in 
the Pacific and only three in the Atlantic. This was 
the smallest monthly number of U-boats sunk in the 
Atlantic since January 1942. Two of these U-boats 
were sunk in the Northern Transit Area-East and 
one in the Biscay-Channel Area. 

The main reasons for the meager results achieved 
by the Allies in sinking U-boats during these three 
months, September through November, was the ex- 
treme caution displayed by the U-boats as the average 
U-boat sank less than 1/10 of a ship per month at 


sea. Increased experience in the use of Schnorchel 
enabled the U-boats to avoid Allied air patrols by 
remaining submerged for prolonged periods and 
lying in wait for Allied convoys in focal areas in- 
shore. Then, the U-boats developed their bottoming 
tactics in inshore waters, where wrecks and non-sub 
contacts were abundant and where the high rever- 
beration background tended to drown out weak 
Asdic echoes. In addition, the necessity for the use 
of anti-Gnat noisemakers by Allied ships made Asdic 
detection more difficult. 

The experience during these three months indi- 
cated to the enemy that U-boats could again operate 
m inshore waters, with Schnorchel, without suffer- 
ing undue losses. This, in effect, made unnecessary 
the previous long voyages to distant areas which had 
been made in order to avoid the heavy Allied air 
coverage in the North Atlantic. The enemy, there- 
fore, had by the use of Schnorchel overcome to some 
extent the great strategic disadvantage resulting from 
the loss of his Biscay bases. The use of Schnorchel 
enabled U-boats to proceed to and from their bases 
in Norway and the Baltic via the Faeroes-Shetland 
passage instead of the more circuitous passage south 
of Iceland. The U-boats could operate effectively in 
all areas of the North Atlantic, particularly in the 
waters around England, where the high density of 
important shipping provided an attractive target 
near the U-boat bases. 

December 1944 witnessed the beginning of a steady 
increase in the number of U-boats at sea in the At- 
lantic. Coincident with the German land offensive 
on the Western Front (Battle of the Bulge), about 
five U-boats commenced operations in the central 
English Channel in an attempt to impede the flow 
of troops and supplies to the Allied armies in Europe. 
Seven ships and one escort were sunk by U-boats in 
the Biscay-Channel Area during December, with 
most of the damage being done toward the end of the 
month by two U-boats, both of whom escaped to tell 
of their success. During the same period an abortive 
offensive was launched by a flotilla of midget U-boats 
(Biber) against convoys in the Scheldt Approaches, 
off the Dutch coast. Only one ship was sunk while 15 
midget U-boats were sunk or captured. 

U-boat activity in other areas was slight during 
December as the world-wide shipping losses to 
U-boats totalled only nine ships of 59,000 gross tons. 

-An independent ship was sunk in the Gulf of Maine 
on December 3 by the same U-boat that landed two 




70 


SCHNORCHEL U-BOATS OPERATE IN BRITISH HOME WATERS 


enemy agents on the coast of Maine on November 29. 
The other ship was sunk in the Pacific off the south- 
east coast of Australia bv a German U-boat, the first 
sinking in these waters since May 1943. 

Nine U-boats were sunk during December, one in 
the Indian Ocean and eight in the Atlantic. Six of 
the eight U-boats sunk in the Atlantic were lost in 
the waters around England, particularly in the Bis- 
cay-Channel Area and the Northern Transit Area- 
East. One of the three U-boats in the Biscay-Channel 
Area ran aground on Wolf Rock due to navigational 
difficulties. 

U-boat activity continued to increase during Janu- 
ary 1945 as 1 1 ships of 57,000 gross tons were sunk by 
U-boats. All 1 1 of these ships were sunk in the Atlan- 
tic by U-boats operating in inshore waters. In United 
Kingdom waters, there were no sinkings in the Eng- 
lish Channel, although U-boats were still patrolling 
there, and the center of activity shifted to the Irish 
Sea, where six ships were sunk during the month by 
U-boats which had penetrated into the region, al- 
though it had long been considered safe from enemy 
attacks and had been used for training. 

The other inshore areas of U-boat activity in Jan- 
uary were off Halifax, where four ships were sunk 
and another damaged, and the western approaches 
to Gibraltar, where one ship and an escort vessel 
were sunk and another ship damaged. Three of the 
four ships sunk off Halifax were torpedoed on Janu- 
ary 14, during daylight, out of a convoy nearing port. 
The escorts were not successful in locating the 
U-boat. 

The number of U-boats sunk monthly reached a 
low for this period as only six U-boats were sunk 
during January, one in the Pacific and five in the 
Atlantic. All of these kills were made by surface craft 
during the latter half of the month. Not a single 
U-boat was sunk by aircraft during January 1945. 
The first of these sinkings occurred on January 16, 
when a U. S. destroyer escort task group sank a 
weather-reporting U-boat in the Northwest Atlantic 
Area. Using information based on the U-boat plot, 
the group departed from the Azores for the general 
area of the last fix. An HF/DF fix, at a range of 10 
miles, further localized the probable area of the 
U-boat and the hunt got underway. Some 2i/^ hours 
later, sonar contact was gained, and one Hedgehog 
attack and five Mk 8 depth-charge attacks resulted 
in the sinking of the U-boat. 

The other four U-boats sunk in the Atlantic dur- 


ing January were all sunk in United Kingdom coastal 
waters by British surface craft. Three of these kills re- 
sulted from Asdic contacts made shortly after ships 
were torpedoed. They were particularly encourag- 
ing in that they represented some evidence that ships 
were finally learning how to attack U-boats operating 
in inshore waters, after months of disappointing 
patrols in generally difficult conditions. 

In addition to these successes, indirect but effec- 
tive anti-U-boat operations were carried out by the 
Russian armies during January. By overrunning 
East Prussia, they threatened the important working- 
up bases in the eastern Baltic, and by entering Silesia 
they paralyzed part of the elaborate organization for 
the construction of the new type U-boats. It is esti- 
mated that the yards at Danzig alone produced al- 
most a third of the new Type XXI prefabricated 
U-boats. Not only were the Norwegian and western 
Baltic ports severely taxed by assuming the addi- 
tional burden of handling surface vessels and U-boats 
previously based in the eastern ports, but the con- 
centration of all U-boat facilities in the western 
Baltic made them much more vulnerable to Allied 
air attacks. 

The U-boats were more aggressive in February 
1945 and sank 15 ships of 65,000 gross tons. One ship 
was sunk in the Indian Ocean, off the west coast of 
Australia, and the other 14 ships were sunk in the 
Atlantic, the peak monthly score for this period. 
With the land warfare in Europe approaching Ger- 
many, the enemy’s objective seemed to be to sink 
the maximum shipping in the short time remaining. 
Consequently, the major U-boat effort was concen- 
trated in British coastal waters, where nine ships 
were sunk during the month, mainly in the south- 
west approaches to the English Channel and in the 
North Sea off the east coast of England. The five 
ships sunk in the remainder of the Atlantic included 
two in the Barents Sea Area, one near Iceland, one 
near Gibraltar, and one in the Southeast Atlantic by 
a U-boat homeward bound from the Indian Ocean. 

The number of U-boats sunk monthly increased 
sharply as 19 U-boats were sunk in February, indi- 
cating that the kills near the end of January initiated 
a new period of good hunting. Twelve of the 14 
U-boat kills in the Atlantic took place in the waters 
around England, but there was a shift of activity to 
the northward. The Tenth Escort Group carried out 
a particularly successful patrol in the area between 
the Shetlands and the Faeroes against U-boats on 


U-BOAT OFFENSIVE 


71 


passage to their operational areas, sinking three of 
them. These three attacks all resulted from initial 
Asdic contacts on submerged U-boats. Ten of the 14 
kills in the Atlantic were made by surface craft as 
Asdic was becoming much more effective against 
U-boats in shallow inshore waters due to the in- 
creased experience of Allied ships under these con- 
ditions. The other two U-boat kills in the Atlantic 
occurred in the Barents Sea Area and off Gibraltar. 

Five Japanese U-boats were sunk during February 
in the Pacific, where the U. S. submarine USS Batfish 
turned in a record performance by sinking three 
U-boats in four days. These were torpedoed north of 
Luzon between February 9 and 12. In each case, the 
Batfish detected Japanese radar signals on her search 
receiver [APR] and then homed on the U-boat. 

The number of U-boats at sea in the Atlantic in- 
creased sharply in March 1945, averaging over 50 for 
the month. Most of these were concentrated in the 
waters around England, but there were some signs of 
a shift of U-boat activity to deeper waters in the 
Atlantic, possibly indicating that the enemy had ap- 
preciated that some escorts had been removed from 
ocean convoys to operate in inshore waters. There 
was also some evidence that Type XXIII U-boats 
operated in the North Sea, off the east coast of 
England. 

Despite the increase in the number of U-boats at 
sea, the world-wide shipping losses to U-boats stayed 
at the same level in March 1945 as only 13 ships of 
65,000 gross tons were sunk by U-boats, all in the 
Atlantic. Nine of these ships were sunk in the waters 
around England, seven of them in the Biscay-Chan- 
nel Area. Two ships were sunk in the Barents Sea 
Area and another two were sunk in the Brazilian 
and Caribbean Areas by a U-boat homeward bound 
from the Indian Ocean. Midgets probably sank 
another three ships in the North Sea Area. 

The number of U-boats sunk during March con- 
tinued high as 19 were destroyed, 2 in the Pacific and 
17 m the Atlantic. Four U-boats were destroyed by 
our raids in German ports and one was sunk in the 
Canadian Coastal Zone by a U. S. destroyer escort 
killer group. The other 12 U-boats were sunk in the 
waters around Great Britain; one by aircraft, two by 
mines, and nine by surface craft. The Twenty-First 
Escort Group turned in a notable performance by 
sinking three U-boats in four days, north of Scotland. 

In April 1945, shipping casualties were of the same 
order of magnitude as in March, as 13 ships of 73,000 


gross tons were sunk by U-boats. All of these ships 
were sunk in the Atlantic: two off Cape Hatteras in 
Eastern Sea Frontier, one in Kola Inlet in the Barents 
Sea Area, and the other ten in coastal waters around 
England, mainly in the Channel Area. 

The tempo of U-boat operations in April gave no 
indication that the end of the war was at hand. With 
remarkable determination the enemy maintained 
his U-boat offensive in inshore waters to the very 
end of the war. No relaxation of effort or hesitation 
to incur risk was apparent until the German sur- 
render on May 8, 1945. A U. S. cargo vessel was sunk 
off Rhode Island on May 5, but a U. S. destroyer 
escort task group gained sonar contact later that 
night and destroyed the U-boat. On May 7, a U-boat 
sank two merchant ships from a coastal convoy near 
the Firth of Forth. These were the last merchant 
ships sunk by U-boats in World War II, as Japanese 
U-boats did not sink a single merchant ship between 
V-E Day, May 8 and V-J Day, August 14, 1945, while 
six Japanese U-boats were sunk in that interval. 

The 36 U-boats sunk during April 1945 made the 
highest monthly score of the last period. Five of these 
U-boats were sunk in the Pacihc and the other 31 
were sunk in the Atlantic, with surface craft account- 
ing for 19 kills, the highest monthly score of the war 
for them. Twenty-two of these U-boats were sunk in 
the waters around England, three in the Barents Sea 
Area, two along the east coast of the United States, 
and four in Northwest Atlantic Area. These four 
U-boats were part of a group of six (Group Seawolf) 
engaged in a joint westward sweep of North Atlantic 
convoy lanes while en route to operations off the U. S. 
coast. Escorts of U. S. escort carrier task groups that 
conducted a barrier patrol which intercepted this 
group of U-boats were responsible for these four kills. 
On the last day of April, a U. S. Navy MAD-equipped 
plane sank a U-boat in the Biscay-Channel Area with 
retro-bombs. 

During the hrst week of May, aircraft operated 
with great effectiveness in Danish waters against 
U-boats attempting to escape to Norway, sinking 
about ten of them with rockets and gunfire. In addi- 
tion to the kills enumerated above, it is estimated 
that the heavy bombing raids on German ports in the 
Baltic probably accounted for the destruction of 
over 25 U-boats in port during the last month of the 
war. 

On May 4 a short signal was transmitted on all 
U-boat frequencies. By this signal, Doenitz had 


72 


SCHNORCHEL U-BOATS OPERATE IN BRITISH HOME WATERS 


ordered his U-boats to cease hostilities. In an Order 
of the Day issued at the same time, he explained that 
a crushing superiority had compressed the U-boats 
into a very narrow area and that the continuation 
of the struggle was impossible from the bases which 
remained. By May 31, 49 U-boats had surrendered 
at sea leaving a small number unaccounted for. The 
last of these to turn up was U-977 which surrendered 
at Buenos Aires on August 17, 1945, after the sur- 
render of Japan. In addition to the U-boats which 
surrendered at sea, huge numbers were captured in 
ports or scuttled. 

The end of the war thus saw the U-boat fleet held 
in check but still carrying out operations on a major 
scale. It had never been driven off the seas and might 
well have increased substantially in numbers and 
power, had the war been extended for any appreci- 
able period. New-type U-boats were just coming into 
service at the end of the war. About six Type XXIII 
U-boats had operated with fair success off the east 
coast of the United Kingdom and at least that num- 
ber of Type XXI U-boats had reached ports in Nor- 
way with the intention of sailing in the immediate 
future for offensive operations. One is believed to 
have actually started on patrol, but it was forced to 
turn back due to some failure in equipment. The 
U-boat was by no means eliminated as a weapon of 
war when Germany surrendered, and the new types 
which Germany was introducing would have raised 
serious problems for the Allies had they ever reached 
large-scale use. 

7 2 COUNTERMEASURES TO THE 

U-BOAT 

Convoys 

Shipping in convoy during this last period was 
safer than in any previous one. Although the number 
of convoyed ships at sea was at its peak, only six ships 
were sunk monthly by U-boats. Of the 1 100 ships that 
sailed monthly in the North Atlantic convoys, only 
about one was sunk monthly by U-boats, usually in 
the vicinity of England. The loss rate was therefore 
less than 1/10 of 1 per cent. 

The proportion of shipping sunk by U-boats, which 
was in convoy when sunk in contrast to independent 
sailings, increased to about 55 per cent during this 
period. This increase was due to the fact that 
U-boats, operating on Schnorchel in inshore waters. 


found it desirable to operate in areas of high ship- 
ping density, that is, along the convoy lanes. The 
experience during this period differed from the past 
in that most of the shipping losses occurred at the 
terminals of the convoy routes, instead of in the 
middle. 

The principle of defense in depth contributed 
greatly to the safety of convoyed shipping, particu- 
larly in the case of the North Russian convoys. Dur- 
ing this last period, there were often sufficient ships 
available so that, in addition to the close escort, 
pickets could be stationed in an outer screen. Pickets 
could intercept surfaced U-boats, investigate surface 
ships, divert neutral ships, and give the Escort Com- 
mander timely warning of impending dangers. Air 
patrols operated in the zone beyond the pickets. 

The largest convoy of the war, HXS 300, consist- 
ing of 167 ships and seven mid-ocean escorts, sailed 
in July 1944. With 19 columns, this convoy had a 
front of some nine miles. The ships in the convoy 
carried over a million tons of cargo to England. The 
convoy arrived safely and the fact that it was not 
attacked may well have been due to the vigorous 
search by aircraft of the two MAC ships for the only 
U-boat reported in the vicinity. 

Early in September 1944, following the transfer of 
the U-boats from the Biscay base to Norwegian ports, 
the Allies rerouted the North Atlantic convoys 
around the south of Ireland, through St. George’s 
Channel. Although not shortening the voyage be- 
tween New York and Liverpool to any appreciable 
extent, the southerly route did facilitate the joining 
and splitting of sections from and to the south coast 
of England and the Continent. It also got further 
away from both the U-boats based in Norwegian and 
German ports and the rough weather of the higher 
latitudes. Convoy CU 37 was the first to sail to an 
Atlantic port on the Continent, as one section ar- 
rived in Cherbourg on September 7. Antwerp was 
opened to Allied convoys on November 28. 

The increased safety of convoys in the Atlantic 
enabled the Allies to make another change that 
would quicken the flow of shipping. Towards the 
end of September 1944 the sailing interval for the 
HX and ON convoys was reduced to 5 days and the 
slow SC and ONS convoys were started again. This 
meant that the convoys would not be as large as they 
had been in the previous months, but there would 
be more of them. The time spent in port by ships 
waiting for convoys was cut materially by this change. 




COUNTERMEASURES TO THE U-BOAT 


73 


Similar steps were taken in the East Atlantic in 
November 1944. A single escort sailed with convoys 
between Gibraltar and the United Kingdom, al- 
though additional protection was given at both ends. 
This more or less restored the arrangement which 
had been in force from the outbreak of the war until 
the middle of 1941. Shipping between Gibraltar and 
Freetown was permitted to sail independently if 
there were no U-boats along the route. In the Medi- 
terranean convoying was abandoned except for 
troopships and local convoys near Italy and Greece. 

In the last month of the war, April 1945, the aver- 
age number of ships at sea in the Atlantic reached a 
maximum of 1400, about double the number in the 
early months of 1943. About two-thirds of these ships 
were in convoy. Shipping in the Pacific also reached 
a new high in April, as there were about 900 ships at 
sea, with about one-third of them in convoy. 


Most of the U-boat activity during this period was 
concentrated in the waters around England and con- 
sequently aircraft under Coastal Command opera^ 
tional control played the major part in the offensive 
against the U-boats. At the end of the war. Coastal 
Command had more than 1100 planes under its con- 
trol, more than six times the number available at the 
start of the war. The invasion battle started in May 
1944, when U-boats left from Norwegian ports to re- 
inforce the Biscay ports for the attack on the huge 
amount of Allied shipping which would have to be 
used for the build-up of the Allied beachhead. Very 
few U-boats got through, as aircraft sank 17 U-boats 
and damaged 1 1 others in Norwegian waters between 
mid-May and the end of July. Thus a depleted 
U-boat fleet was left to execute the plan to attack the 
invasion traffic. 

As soon as the invasion had started, the U-boats 
headed for sea, staying on the surface and fighting 
back against aircraft with their automatic 37-mm 
gun. Coastal Command was ready, and within five 
days the enemy lost six U-boats while five were seri- 
ously damaged. The hectic few days after D-day pro- 
duced one of the outstanding achievements of the 
war, when a Liberator sank two U-boats at night 
within half an hour. After the first week of the inva- 
sion, the enemy abandoned his ideas of staying on the 
surface and the all-Schnorchel era had begun. But 
the beachhead was already secure and the U-boat 


fleet badly mauled. Between mid-May and the end 
of July Coastal Command aircraft sank 23 U-boats, 
shared two kills with surface craft, and damaged 25 
U-boats. The price paid for the success was 31 air- 
craft lost as a result of enemy action and 17 lost as a 
result of operational hazards. 

Once the invasion beaches were secure, it became 
Coastal’s job to protect shipping until victory was 
finally won. It was difficult to detect Schnorchel at all 
and even after it was detected, the probability of at- 
tacking it successfully was lower than that for an at- 
tack on a surfaced U-boat. All available aircraft were 
employed to hunt the Schnorchel and, although the 
number of kills was somewhat disappointing, only a 
few ships were sunk in inshore waters. Thus the 
many hours of flying often without the consolation 
of a sighting were not wasted. 

With the reduced opportunities to kill U-boats, 
Coastal Command began to look further afield. Dur- 
ing 1945, there were a number of anti-U-boat sorties 
in the Skagerrak, Kattegat, and the western Baltic. 
Liberators at night and rocket-fitted Mosquitos and 
Beaufighters during the day carried the war right 
into the U-boats home waters. Numerous attacks 
were made, some of them on the new type U-boats 
which were proceeding to Norwegian ports prior to 
setting out on operations. The outstanding effort was 
a strike in the Kattegat by Mosquitos, which re- 
sulted in the sinking of three U-boats. In the final 
days of the war, the last real action was seen when 
U-boats began to evacuate north German ports and 
run for Norway. Many attacks were made and about 
ten U-boats are believed to have been sunk by air- 
craft in the first week of May 1945. 

During the entire period aircraft operating under 
Coastal Command made about 38 sightings and 24 
attacks monthly, about the same as in the previous 
period although the concentration of U-boats in 
British waters was much higher in the last period. 
The use of Schnorchel resulted in a reduction in the 
lethality of aircraft attacks, as only 18 per cent of 
them resulted in the sinking of a U-boat (about 25 
per cent in previous period) and about 35 per cent of 
the attacks resulted in at least some damage to the 
U-boat. About half of the aircraft kills made during 
this 11-month period occurred in the first and last 
months (June 1944 and April 1945), when a high pro- 
portion of the U-boats were found on the surface. 

Operational results during the last period indi- 
cated that Schnorchel was a most effective counter- 


74 


SCHNORCHEL U-BOATS OPERATE IN BRITISH HOME WATERS 


measure to Allied air power. Most of the U-boats 
were equipped with Schnorchel, fitted with a drum- 
shaped aerial (Runddipol) which was pressure-tight, 
but gave warning only of meter radar. The primary 
effect of Schnorchel was to cut visual and radar 
ranges from Allied aircraft by a factor of about 2 or 3. 
Toward the end of this period the Germans devel- 
oped an effective anti-radar rubber-like covering for 
the Schnorchel, which was supposed to cut Allied 
microwave radar ranges on Schnorchel by another 
factor of 3. The ranges at which many radar con- 
tacts would first be made were such as to place them 
within the sea-return zone of Allied radar sets, where 
they were missed entirely. It has been estimated that 
the number of potential Schnorchel contacts missed 
at short ranges is larger than the total number of 
Schnorchel contacts made at all ranges. 

These advantages of Schnorchel enabled U-boats 
to operate over long periods of time in restricted 
waters, with reasonable safety from aircraft, by re- 
maining bottomed most of the time and coming to 
Schnorchel depth only to recharge batteries. The 
effectiveness of aircraft hunts was also reduced as the 
Schnorchel increased the submerged speed of U-boats 
for prolonged periods from about 2 or 3 knots to 
about 6 knots. 

Schnorchel, however, also had its disadvantages. 
The offensive power of U-boats was greatly reduced 
as they were forced to give up the great mobility of 
surfaced operations for the relatively slow speeds of 
Schnorchel operation. The efficiency of the periscope 
watch was impaired when the U-boat was at Schnor- 
chel depth, and the noise of the engines rendered the 
hydrophones practically useless. In addition, pro- 
longed use of Schnorchel undoubtedly increased per- 
sonnel fatigue, as a result of varying air pressure, 
occasional fumes, and the more careful attention re- 
quired to maintain depth control. 

Despite these disadvantages, the U-boats preferred 
the feeling of security which the Schnorchel gave 
them to the alternative of operating on the surface, 
and depending on their new directional microwave 
search radars for warning of aircraft. With the in- 
creased use of Schnorchel, the number of U-boat 
contacts per 1000 flying hours steadily decreased and 
the ratio of Schnorchel contacts to all contacts stead- 
ily increased, passing the 50 per cent mark in De- 
cember 1944. Visual search became relatively more 
productive than radar search as occasional sightings 
at relatively long ranges were made on the exhaust 


smoke or wake that sometimes accompanied the 
Schnorchel. 

The Allies were not able to produce any really 
effective countermeasures to the Schnorchel by the 
end of the war. Increased stress was placed on the use 
of binoculars in visual search and the most favorable 
altitudes for the detection of Schnorchel were deter- 
mined. Tactical doctrines were modified to take ac- 
count of the decreased sweep widths of both radar 
and visual search. Modifications, such as the fast 
time constant [FTC] circuit, were made to Allied 
radar sets to improve the efficiency of contact. The 
FTC circuit acted as a discriminator against sea 
return, thereby making it easier to distinguish the 
Schnorchel blip. New high-power narrow-beam 
short-pulse radar sets were designed to provide better 
resolution in search for small targets. Tests indicated 
that the new radar sets (AN/APS-3 and AN /APS- 15), 
which operated in the X-band (3-cm) and had sharp 
beams, were better at detecting Schnorchel than the 
earlier longer-wave-band radar sets. 

Aircraft used sono-buoys more frequently towards 
the end of the war in order to take advantage of the 
high noise output of Schnorchelling U-boats. Sono- 
buoys had been used successfully in a number of at- 
tacks by aircraft and escorts of U. S. escort carrier task 
groups and analyses indicated that in over 50 per 
cent of the cases in which sono-buoys were dropped 
following visual contacts on U-boats a sono-buoy con- 
tact was obtained. 

Scientific and Technical 

The Allies continued to perfect their equipment 
and tactics during this last period. More useful and 
flexible retiring search plans were developed for sur- 
face craft. Operation Observant and similar search 
plans, based on the most probable location of the 
U-boat, provided ships with the means of regaining 
contact with a U-boat once the general location was 
known. An analysis of surface craft hunts indicated 
that when the correct plan was used, contact was re- 
gained in 44 per cent of the cases, while the incorrect 
plan led to success in only 28 per cent of the cases. 

The U. S. Navy continued to improve its sonar 
equipment as experimental work was done with 
maintenance of deep contact feature [MDC] and 
depth-determining gear. By the end of the war a new 
fast sinking influence depth charge had been de- 
veloped, the Mark 14, which was considered to have 


COUNTERMEASURES TO THE U-BOAT 


75 


higher probability of doing lethal damage to a sub- 
merged U-boat than other depth charges. This charge 
was designed to fire at the nearest point of approach 
to a U-boat by the change of frequency between the 
reflected signal and the supersonic signal-emitted by 
the depth charge. 

An analysis was made of the operational results 
obtained during 1943 and 1944 by British ships using 
depth charges, Hedgehog, and Squid. It indicated 
that a single depth-charge pattern had about a 5 per 
cent chance of sinking a U-boat and a Hedgehog pat- 
tern at least a 15 per cent chance, while the Squid at- 
tacks averaged about a 20 per cent chance of success. 
The double Squid pattern showed promise of being 
the most lethal weapon against U-boats, but this was 
based on a small number of attacks. 

The greatest technical effort of the Allies, however, 
was spent on the effort to develop satisfactory means 
for the detection of Schnorchel. As previously men- 
tioned, two main lines were followed: (1) the im- 
provement of radar performance by choosing a de- 
sign effective against small targets, and (2) the im- 
provement of sonar detection, in particular sono- 
buoys for detection from aircraft. The chief modifi- 
cation involved was increase in the operating life of 
the buoys to reduce the number that had to be em- 
ployed. In addition, directional sono-buoys were de- 
veloped, which gave a more accurate submarine posi- 
tion, but these did not see operational use. 

The U-boat command again was prolific in devel- 
oping new technical equipment for the U-boats in an 
effort to stave off the impending defeat. During 1944 
the Germans introduced a new type of gear [LUT] 
on their torpedoes which enabled the line of advance 
of the torpedo, when zigzagging, to be pre-set at any 
angle from its straight run. This gear also enabled 
the mean speed of advance to be pre-set at will. At the 
end of the war the Germans were developing a new 
type of homing torpedo (Geier) which omitted super- 
sonic signals and homed on the reflected echoes from 
the target. 

During this period, the Germans modified their 
740- and 1200-ton U-boats to enable them to dive as 
quickly as the 500-ton U-boats. The most distinctive 
feature of these modified U-boats was the narrow 
cut-away deck forward. 

Toward the end of the war, there were some indi- 
cations that the Germans had developed an ultra- 
high-speed method of communication (Kurier) using 
an attachment to the normal U-boat transmitter. It 


appears that the message was recorded on a magnetic 
tape which was then run through at high speed. This 
method of communication would counter Allied use 
of HF/DF. 

Early in 1944 the U-boat command had come to a 
clearer appreciation of the true situation with regard 
to the Allied use of radar. They realized that they 
needed an improved search receiver against S-band 
(10-cm) radar which would not only give ample warn- 
ing but would provide a margin of sensitivity against 
inevitable losses of efficiency under operational con- 
ditions. The enemy had also learned of X-band 
(3-cm) radar from a crashed H 2 X blind bombing 
plane at the beginning of 1944 and the development 
of X-band search receivers was started by the Ger- 
mans before Allied use of X-band radar in U-boat 
search had produced many results. 

The German solution to these problems appeared 
in the late spring of 1944 in the form of the “Tunis” 
search receiver. Tunis consisted of two antennas, the 
“Mucke” horn for X-band radar and the “Cuba la” 
(Fhege) dipole and parabolic reflector for S-band 
radar, fitted with an untuned crystal detector and 
connected to a Naxos 2 amplifier. Bearings were 
taken by rotating the DF loop on which the antennas 
were mounted. The X-band Mucke horn had a beam 
width of about 15° so that bearings on 3-cm radar 
were quite accurate. The S-band Fliege antenna de- 
tected 10-cm radar transmissions through a sector of 
about 90°. 

The chief feature of this equipment was the direc- 
tional antennas, which gave increased sensitivity and 
range. In order to obtain the necessary sensitivity 
with these aerials, the Germans had to sacrifice the 
desirable property of all-around looking and the 
aerials had to be continuously rotated by the bridge 
watch. The units still had to be dismounted and 
taken below on submergence, and so could not be 
used on Schnorchel. 

Allied tests on captured equipment indicated that 
Tunis was simple to operate and dependable under 
normal operational conditions. These tests indicated 
that expected operational ranges on Allied radar sets 
would vary from about 20 miles for planes at 500 feet 
altitude to about 40 miles for planes flying at 2000 
feet. These expected operational ranges of Tunis 
were greater than the corresponding average radar 
ranges, both for S-band and X-band, on surfaced 
U-boats. It was concluded that the Tunis search re- 
ceiver was apparently a simple, efficient, and suc- 




76 


SCHNORCHEL U-BOATS OPERATE IN BRITISH HOME WATERS 


cessful countermeasure to both S-band and X-band 
radar. 

The Tunis search receiver was used very infre- 
quently by U-boats during this period and it is quite 
possible that the Germans were not aware of how 
successful it could be against Allied radar. Tunis 
was developed shortly after Schnorchel had been 
fitted to the U-boats and never did receive a fair trial 
in actual operations. The U-boats apparently did not 
have much faith in their technical experts, who had 
fumbled so badly in 1943, and preferred the security 
offered by Schnorchel, despite its limitation for the 
offensive, to the risk of operating on the surface and 
depending on Tunis for early warning of Allied air- 
craft. Then again Tunis offered no protection 
against possible new Allied radar sets on different 
frequencies or against Allied aircraft not using radar 
at all, while Schnorchel did. It seems quite likely that 
the U-boats would have sunk considerably more ship- 
ping if they had operated on the surface with Tunis, 
but the desire for safety was predominant and 
Schnorchel offered the better prospects for that. 

All the centimeter aerials produced by the Ger- 
mans had the disadvantage that they could not with- 
stand submergence. The production of a suitable 
pressure-tight aerial, urgently needed for use on 
Schnorchel, presented great technical difficulties 
which were not completely solved at the end of the 
war. The only search receiver aerial that was fitted 
on Schnorchel was the old standard Runddipol type, 
which could detect only meter radar. There w^re 
some signs that the Germans were developing a pres- 
sure-tight aerial at the end of the war, suitable for 
both S-band and X-band radar detection and incor- 
porating an infrared receiver as well. 

During this period the Germans introduced a 
variety of midget U-boats {e.g.: Seehund, Molch, 
Biber, Hecht, and Marder), piloted by one or tw^o 
men from a pressure-tight control position and cap- 
able of complete submergence. The successful attack 
by British midget submarines on the Tirpitz on Sep- 
tember 22, 1943, stimulated German interest in these 
craft. The flotillas operating these midget U-boats 
were not branches of the U-boat arm of the German 
Navy but form part of an organization known as the 
Small Battle Units Command [KDK]. 

The Seehund (Type XXVII) was probably the 
most formidable of the midget U-boats. It was pre- 
fabricated, about 39 feet long, and displaced 16 tons. 
It carried two torpedoes and a two-man crew. The 


surface speed w^as about 6 knots and submerged speed 
about 3 knots. The surface range w^as about 275 
miles, the endurance of the crew^ about three days, 
and the diving depth about 100 feet. It was fitted 
with a periscope. 

Midget U-boats were used primarily in the inva- 
sion area and w^re generally ineffective, inflicting 
very little damage on Allied shipping while suffer- 
ing heavy losses. This may have been due partly to 
the fact that they were rushed into battle before they 
w^ere perfected and before their crews w^ere properly 
trained. Their small size made it more difficult to 
detect them but they w^re extremely vulnerable to 
depth-charge attack. They w^ere relatively slow and 
unhandy, with a limited operational range, and only 
suitable for attacking merchant ships proceeding 
slowly in calm waters. It is estimated that about 80 
midget U-boats were sunk or captured betw’een De- 
cember 23, 1944, wTen they started operations in the 
Channel Area, and the end of April 1945. 

The heavy losses sustained by U-boats in the Battle 
of the Atlantic forced the enemy to undertake the 
gigantic task of building a complete new frontline 
U-boat fleet. Though faced w4th imminent invasion 
and with a great shortage of manpow^er, Germany 
employed a very large number of their skilled wwk- 
men in the construction of prefabricated U-boats. 
This is a measure of the importance that the enemy 
attached to U-boat warfare and of the hopes which 
he entertained for success in a new campaign. 

The Type XXI U-boat w^as designed for high sub- 
merged speeds, primarily to get into a favorable at- 
tack position. Originally intended for turbine drive, 
the high speed hull design for the Type XXI w^as 
completed before the 'Walter propulsion unit w^as 
ready and it was actually equipped wdth extra large 
batteries to give the high maximum submerged 
speed of 15 to 18 knots. Its submerged endurance at 
a speed of 10 knots was about 11 hours. The type 
XXI U-boat w^as about 250 feet long and its standard 
displacement about 1600 tons. The large torpedo 
room w4th 6 bow tubes was an answ’er to the demands 
for increased armament. The U-boat carried a crew 
of 57 men and 20 torpedoes. Stern torpedo tubes w^ere 
sacrificed to obtain greater speed. Silent-running 
speeds of about 5 knots were obtained on electric 
motors when submerged. An improved extensible 
type of Schnorchel was fitted. The Type XXI U-boat 
used diesel propulsion on the surface and had a 
maximum speed of 15 knots. It was a true ocean- 




COUNTERMEASURES TO THE U-BOAT 


77 


going U-boat and its surface endurance was esti- Three Type XVII-B U-boats with turbine drive 
mated to be greater than that of a 740-ton U-boat. were completed before the end of the war. These 
The Type XXIII U-boat was developed on re- were built for experimental purposes to obtain infor- 
quirements from the Mediterranean U-boat com- mation on tactics and improvements for the Type 
mand for inshore waters and short cruises. These XXVI U-boat, which was to be the perfect U-boat, 
characteristics were also useful for operations in the incorporating all the advantages of earlier experi- 
mvasion area. The Type XXIII U-boat was about ence and the extremely high speeds available with 
114 feet long and its standard displacement was the Walter propulsion units. The high speed ob- 
about 230 tons. It had two bow tubes and carried tained from turbines using hydrogen peroxide fuel 
only two torpedoes. The total crew consisted of only was of limited duration and was only to be used in 
14 men. Its maximum surface speed was 10 knots and attacks or other emergencies. A diesel for cruising at 
Its maximum submerged speed 12 knots. The sub- Schnorchel depth and a small battery and electric 
merged endurance at 10 knots was about 41/2 hours, motor for quiet running were to be used for all other 
Both Type XXI and Type XXIII U-boats were purposes. There was no expectation of ever operat- 
btiilt by a system of prefabrication which fell into ing these U-boats on the surface, 
four stages. Basic parts were manufactured at a num- The Type XXVI U-boat was intended to be about 
ber ol widely dispersed factories situated along Ger- 184 feet long and to carry a crew of 37 men. The 
many’s inland waterways. Sections of the hull were maximum submerged speed on turbines was to be 24 
assembled at a number of shipyards and sent from knots with an endurance of about 6 hours at that 
them to certain key yards for welding into complete speed. Ten torpedoes were to be carried in four bow 
U-boats, which were then fitted out under covered and six side torpedo tubes. Sound gear was to be used 
shelters. The final assembly yards for the Type XXI to direct torpedo fire from a submerged position and 
U-boats were Hamburg, Bremen, and Danzig. all ten torpedoes could be fired in one salvo. 

Speer’s dispersal of his organization did much to It should be stressed at this point that no Type 
defeat Allied bombing, but the indirect results of XXVI U-boats were ever built and that no Type 
Allied raids were serious. The German transport sys- XXI U-boats ever operated at sea before V-E Day. 
tem was disrupted by Allied bombing of communi- The conclusions developed in this history of World 
cations and this did much to set back the time sched- War II do not necessarily apply to these high sub- 
ule for the construction of these new U-boats. The merged speed U-boats, 
first prefabricated U-boat was completed in June 
1944 and by the time the war ended, the enemy had 


completed about 120 Type XXI U-boats and about 

fin WTTT TT 1 .1 .1 


Sinkings of U-boats 



78 


SCHNORCHEL U-BOATS OPERATE IN BRITISH HOME WATERS 


Surface craft displaced aircraft as the leading killer 
of U-boats during this period, as most of the German 
U-boats operated on Schnorchel. Allied ships sank 
92 U-boats alone (47 per cent of total) and another 
15 (8 per cent of total) with the help of aircraft. Air- 
craft sank 57 U-boats (29 per cent of total). As most of 
the U-boats operated in inshore waters, carrier-based 
aircraft played a smaller part during this last period 
accounting for only 14 of the 72 U-boat kills in which 
aircraft participated. Allied submarines sank 17 
U-boats (9 per cent of total), most of them in the 
Pacific. The other 15 U-boats (7 per cent of total) 
were lost as a result of mines, marine casualties, and 
scuttling. 

The quality of surface craft attacks reached its 
highest level during this last period as over 30 per 
cent of the attacks were lethal. This percentage 
reached a high of about 50 per cent during the in- 
vasion period, when U-boats first started operating 
on Schnorchel. It dropped to about 20 per cent as 
U-boats perfected their inshore tactics during the 
last quarter of 1944 and then increased again to 
about 30 per cent during the first few months of 1945, 
as British escort groups learned how to deal with the 
bottomed U-boat. 

7 3 SURVEY OF RESULTS 

7.3.1 From the U-boats’ Point of View 

Some idea of the attitude of U-boat crews and of- 
ficers during this last period may be obtained from 
statements made by prisoners of war. Toward the 
end of 1943, many doubts and questions had arisen 
concerning the outcome of the war and the supposed 
superiority of German weapons. These doubts be- 
came stronger when, despite the many promises of 
new weapons made by the U-boat Command, prac- 
tically none were supplied and the U-boats’ situa- 
tion steadily deteriorated. The former enthusiasm 
and confidence of the men in the power of their arms 
and in the competence of their leaders turned into a 
kind of lassitude, and resulted in a mechanical exe- 
cution of commands. The phrase “orders are orders’’ 
characterized the typical state of mind. The majority 
of U-boat officer survivors expressed in no uncertain 
terms the opinion that the U-boat was no longer 
practicable as an offensive weapon in view of the 
effectiveness of Allied antisubmarine measures at 
that time. Despite their state of mind, the U-boats 
fought to the very end of the war with discipline un- 


impaired, and there were no signs of any collapse of 
the German Navy, such as occurred in 1918. 

About 13 U-boats were sunk monthly in the Atlan- 
tic, slightly less than the previous figure of 16. The 
average number of U-boats at sea, however, had de- 
clined to 39 from the average of 61 in the previous 
period. This meant that the average life of a U-boat 
at sea in the Atlantic was only three months, even 
lower than the average life of four months in the 
previous period. Despite the use of Schnorchel and 
maximum submergence tactics, the U-boats found 
operation in inshore waters just as hazardous as they 
had been at the beginning of the war when the U- 
boats had to give up close-in submerged operations. 

The exchange rate in the Atlantic was about the 
same as in the previous period as only 6/10 of a ship 
(3300 gross tons) was sunk by the average U-boat 
before it itself was sunk. U-boats operating in inshore 
waters where there was a heavy concentration of 
shipping were able to sink a little more shipping 
per month at sea during this period but this was 
balanced by the higher loss rate suffered by them. 

The tactics evolved by the Schnorchel U-boat 
toward the end of the war involved practically no 
surfaced operations. In transit, navigation was by 
dead reckoning, radio-navigational fixes off the chain 
of “Electrasonne’’ transmitters and beacons, and also 
by echo sounder and periscope bearings of lights or 
points of land. Schnorchelling was mostly at night, 
for only about four hours in each 24. When Schnor- 
chelling, a U-boat kept all-around periscope watch 
day and night, and constant meter-band GSR watch. 
Diesels were stopped once every 15 to 30 minutes to 
make an all-round hydrophone sweep. Once the 
U-boat was in the patrol area, it was a recognized 
tactic to lie on the bottom of a convoy route and to 
come up only when hydrophone contact was estab- 
lished. For aiming the torpedoes, the periscope was 
still virtually the only instrument available. Salvos of 
curly torpedoes were fired among the merchant ships, 
or else a single acoustic torpedo at either the convoy 
or an escort. The usual methods of evading Allied 
counterattacks were either lying on the bottom, or 
else proceeding at silent speeds of about 3 knots or 
less on electric motors. 

The Schnorchel enabled U-boats to operate in the 
above manner in inshore areas. These U-boats would 
have met certain destruction, had it still been neces- 
sary for tjiem to surface periodically, for a few hours, 
to charge batteries. The problem presented by the 


SURVEY OF RESULTS 


79 


Schnorchel U-boat has been the chief concern of sur- 
face craft, whose main difficulty has been that of dis- 
tinguishing between a bottomed U-boat and a wreck. 
An extensive survey of wrecks in British waters 
helped considerably in solving this problem. 

Schnorchel enabled U-boats to operate in inshore 
waters, but it did not enable them to achieve any sig- 
nificant results. Operational results during the last 
two years of the war indicated that the standard 
U-boat, with or without Schnorchel, could not oper- 
ate successfully against Allied antisubmarine meas- 
ures. It is important to realize, however, that the 
U-boat war was not decisively ended in May 1945. 
Germany had a fleet of about 120 Type XXI U-boats 
read to start operations and was developing the Type 
XXVI U-boat. It is difficult to say whether the high 
submerged speed would have restored the advantage 
to the U-boats and would have enabled them to in- 
flict considerable damage on Allied shipping without 
suffering excessive losses. 


7.3.2 From the Allies’ Point of View 

The Allied and neutral nations lost about 114,000 
gross tons of merchant shipping each month, from 
all causes, during this period. This was the lowest 
average monthly loss of the war and about 25 per 
cent less than in the previous period. The construc- 
tion of new merchant shipping averaged about 850,- 
000 gross tons a month, slightly less than in the 
previous period. Consequently, there was a net gain 
of about 736,000 gross tons of shipping each month, 
or a total increase during this period of about 8,000,- 
000 gross tons in the amount of shipping available. 

Of the 114,000 gross tons of shipping lost monthly 
from all causes, about 84,000 gross tons were lost as 
a result of enemy action. U-boats accounted for 
56,000 gross tons a month, or about 67 per cent of the 
total lost as a result of enemy action. Monthly losses 
to enemy mines rose to 15,000 gross tons (18 per cent 
of enemy action total) as Allied shipping moved 
close to the shores of Europe after the invasion. 
Losses to enemy aircraft dropped to 8000 gross tons 
(9 per cent of enemy action total) a month. The 
other 5000 gross tons (6 per cent of enemy action 


total) sunk monthly were lost as a result of surface 
craft attack and other enemy action. 

By the end of this period. Allied armies had over- 
run Germany, and the surrender came on May 8, 
1945. The Allies had defeated the U-boats in 1943 
and had succeeded in keeping them ineffective there- 
after. It was both an offensive and defensive victory 
as the average U-boat’s lifetime at sea in the Atlantic 
was only three months and it sank only about one- 
half ship before it itself was sunk. The Allies were 
not able, however, to destroy the enemy’s U-boat 
fleet without invasion, as the enemy’s construction 
program had been able to replace all losses. 

The war seems to have demonstrated that the 
standard U-boat could not operate on the surface 
against strong Allied air power. It should be re- 
membered, however, that the U-boats did not oper- 
ate, to any large extent, with a reliable radar or 
search receiver that could detect Allied planes at 
long range. Denied the surface of the sea, the stand- 
ard U-boat, with limited submerged speed and en- 
durance, could not operate effectively against 
strongly escorted Allied convoys. 

The German solution to this problem was the de- 
velopment of the Type XXI and Type XXVl 
U-boats, which had much higher submerged speed 
and endurance and could operate underseas en- 
tirely. These U-boats would be relatively as safe from 
air attack as were the standard Schnorchel U-boats 
and would be much safer from surface craft attack, 
due to their high submerged speeds. They would be 
much more effective against convoyed shipping as 
their high speed would enable them to approach 
and keep up with Allied convoys without surfacing. 
We cannot determine without extensive trials and 
exercises with high submerged speed U-boats just 
how much more effective than the standard types 
they would be. We cannot now determine quanti- 
tatively whether they would be 50 per cent better, or 
ten times as good. From the history of World War II, 
however, we may conclude that these new U-boats 
would have to be about eight times as effective in 
sinking ships and about four times as safe at sea as 
were the Schnorchel U-boats in the last period, in 
order to achieve the same results as were achieved by 
the U-boats during their peak period, January 1942 
through September 1942. 


Chapter 8 

SUMMARY OF ANTISUBMARINE WARFARE 
WORLD WAR II 


8 1 OVERALL RESULTS 

T he opening hours of the war saw the U-boats 
already in position astride the approaches to the 
United Kingdom. Their aim throughout the war 
was to sever the flow of merchant shipping to and 
from Great Britain, and in the attempt the battle 
was carried halfway around the world. The U-boats 
held the initiative from the beginning until their dis- 
astrous defeat in the summer of 1943. Thereafter all 
their efforts were futile, and U-boat warfare, old 
style, was at an impasse when the war ended. What 
the enemy might have accomplished with the new 
U-boats having high submerged speed is conjectural, 
but there can be no doubt that they would have been 
a serious menace. 

The actual achievements of the German, Italian, 
and Japanese U-boats during the war resulted in the 
Allies’ loss of 2753 ships*^ of 14,557,000 gross tons. 
The enemy also suffered considerable losses at sea. It 
is estimated that 733 German, 79 Italian, and 99 
Japanese U-boats were sunk. A total of about 1500 
U-boats were built by the enemy, however, to achieve 
these results. 

There were, therefore, about three Allied mer- 
chant vessels sunk by U-boats for each U-boat sunk, 
about two merchant vessels sunk for each U-boat 
built. In addition, the Allies were forced to maintain 
a large and costly antisubmarine effort which di- 
verted their attention from other phases of the war. 
In these terms the U-boat war was a profitable one for 
the enemy, even though the U-boats were ultimately 
defeated. If German U-boat operations only were 
considered, this conclusion would be greatly 
strengthened. During the period prior to June 1943 

a The figures given in this summary are based on CNO rec- 
ords as of November 19, 1945. They do not agree exactly with 
figures given in other chapters which were prepared earlier. 
Assessments of.attacks, in particular, have been changed some- 
what on the basis of intelligence gained from German sources 
after the German surrender. As is shown in Appendix I, how- 
ever, the earlier assessments as they existed at the end of the war 
were in good general agreement with German records of sub- 
marine losses and therefore provide a fairly reliable basis for the 
discussion given in the other chapters. 


they were, of course, unquestionably successful, 
whereas subsequent operations were an equally com- 
plete success for the Allies. 

82 MAJOR DEVELOPMENTS 

The beginning of the war saw an immediate cam- 
paign of unrestricted U-boat warfare. The British 
had anticipated such a development and put a pre- 
viously planned convoy system in operation within 
a few days. The scale of effort was small, however, as 
there were only a small number of U-boats at sea, and 
the antisubmarine craft available to the Allies could 
give only very weak escort to the convoys. 

The U-boats concentrated their forces in the vicin- 
ity of Britain, making their attacks in daylight from 
periscope depth. Their tactics were highly aggres- 
sive, and each U-boat scored heavily against Allied 
shipping. As a result they exposed themselves to 
counterattack by British surface craft, whose Asdic- 
directed attacks proved to be much more effective 
than the Germans were expecting. Surprised and 
confused by the success of British escorts, the U-boats 
devoted most of their attention to independent ships 
and those in convoy were relatively safe, even though 
weakly escorted. 

In June of 1940 France fell to the German Army. 
The resulting worsening of the British strategical 
situation had a direct effect on the U-boat war. In the 
first place the threat of a seaborne invasion of Eng- 
land confined large numbers of British air and sur- 
face craft to the east coast anti-invasion patrols and 
diverted them from antisubmarine duties. In addi- 
tion, the acquisition of bases on the Bay of Biscay cut 
down on the transit time of the U-boats and allowed 
them to extend their operations farther into the 
Atlantic. 

As a result of the effective counterattacks suffered 
by submerged U-boats, the Germans introduced a 
radical change of tactics. They began to attack on the 
surface at night, a procedure that was characteristic 
of them during most of the war. They capitalized on 
the weakness of British escorts and made many of 


G^ 


MAJOR DEVELOPMENTS 


81 


the attacks on convoys, usually trailing the convoy 
until darkness, then coming in trimmed down on the 
surface for the attack, and retiring on the surface at 
high speed. 

This method was highly successful, and bold indi- 
vidual attacks rolled up large tonnages to the U- 
boats’ credit. The risks were proportionately high, 
however. By such tactics the outstanding U-boat 
aces, Prien, Kretschmer, and Schepke, each amassed 
totals of over 200,000 gross tons, but all three were 
eventually sunk in March 1941. 

One of the most significant countermeasures to 
these surfaced attacks was the introduction of radar; 
a makeshift aircraft set was first fitted to escorts in 
November 1940. Its effectiveness was not great, how- 
ever, during this early period. Various improvements 
in the convoy system were made by the British, in- 
cluding the formation of escort groups. Admiralty 
control of routing, and a wide dispersion of convoy 
routes. As a result, U-boats had greater difficulty in 
attacking convoys. 

Early in 1941 the results of Germany’s U-boat con- 
struction program began to be felt. In 1939 the Ger- 
mans had only a small U-boat fleet, since their hopes 
were for a short war. When the possibility of a long 
struggle with Britain became apparent, building of 
U-boats was given high priority. They were commis- 
sioned at a rate of about 20 a month starting in 1941. 
There was a corresponding increase in U-boat per- 
sonnel which, of course, resulted in a widespread 
lowering in their general experience and capability. 
This was accentuated by the loss of some of the best 
trained crews. 

In the summer of 1941, therefore, the bold tactics 
of individual night surfaced attacks on convoys were 
modified. The policy of wolf-pack attacks came into 
use in an attempt to overwhelm the convoy escorts. 
The procedure was for the U-boat first contacting a 
convoy to withhold attack, trailing the convoy and 
homing other U-boats to the scene so that a number 
could join in the attack, thus capitalizing to the full 
on the opportunity and reducing the danger to the 
attacking U-boats. Complementary to this policy was 
a general movement of U-boats to the west and south 
in an effort to find independent ships. This move- 
ment also gave freedom from the growing air cover in 
the vicinity of Britain. 

As a result of these changes, the British were forced 
to adopt complete end-to-end escort of transatlantic 
convoys. They were aided in this effort both by the 


German attack on Russia in June 1941 which ended 
the threat of invasion of England and released craft 
for antisubmarine operations, and by the entry of the 
U. S. Navy in convoy operations in September. De- 
spite the great increase in number of U-boats at sea. 
Allied losses of merchant vessels did not increase. 

Another change introduced by the British during 
1941 was to have a profound effect on the antisub- 
marine war, in this case the contribution made by 
aircraft. Air patrol had been effective in harassing 
the U-boats from the beginning, but very few suc- 
cessful attacks were made for lack of suitable weap- 
ons. The need for a depth bomb exploding at shal- 
low depths was finally realized, and a 25-foot depth 
setting came into use. With this minor change air- 
craft eventually became the equal of surface craft as 
killers of U-boats. 

With the U. S. entry in the war at the end of 1941 
the scope of U-boat operations expanded rapidly. 
The Germans had a large and rapidly growing 
U-boat fleet, so that they were able to launch a full- 
scale offensive against the weakly protected shipping 
in U. S. coastal waters. They were at first able to 
choose and sink their victims virtually unimpeded, 
and Allied losses reached catastrophic size, 140 ships 
of 698,000 gross tons in June 1942, for example. De- 
fenses were organized, however, which forced an 
eventual withdrawal of the U-boats. With the intro- 
duction of convoying in Eastern Sea Frontier in May, 
losses in that area were reduced, while those in Gulf 
Sea Frontier soared. U-boat successes there were 
short-lived, though the Caribbean remained a soft 
spot throughout the summer of 1942. Nevertheless 
the continued extension of convoying and air patrol 
drove the U-boats out of the coastal areas by fall, and 
they then returned to the North Atlantic. 

During this period U-boat activity in the Eastern 
Atlantic was at a standstill. Consequently British 
forces were free to begin a counteroffensive against 
U-boats in transit from their bases to the operating 
areas in the Western Atlantic. Coastal Command air- 
craft patrolling the Bay of Biscay with radar and 
searchlights inflicted considerable damage on the 
U-boats during the summer, until countered by Ger- 
man search-receiver development. 

As a direct effort to make up the heavy shipping 
losses, the Allies started an intensive building pro- 
gram during 1942. Despite all their efforts, however, 
U-boats sank ships faster than they were built until 
about the end of the year. 




82 


SUMMARY OF ANTISUBMARINE WARFARE, WORLD WAR II 


In October it was evident that the U-boats were 
returning to the North Atlantic in force. Attacks on 
transatlantic convoys were their objective, and they 
operated in the mid-ocean gap which could not be 
reached by land-based air patrol. This gave them 
freedom to operate on the surface and gather very 
large wolf packs— often ten or more and occasionally 
as many as 30 to 40. A concerted attack of this sort 
often led to breaks in the escort formation and dis- 
organization of the defense. 

Nevertheless the vast convoys for the North 
African invasion made their way from Britain with- 
out serious losses because of the complete air cover- 
age provided for them. Routine trade convoys were 
less fortunate as the tonnage lost topped 700,000 in 
November. Against a U-boat fleet which was able to 
maintain about 100 U-boats at sea, even the more 
efficient escorts which were fitted with S-band radar 
were inadequate. Heavy losses continued during the 
winter, mostly in the North Atlantic, but also in 
other widespread areas where diversionary U-boats 
were operating. 

In the spring of 1943 the convoy defenses were 
bolstered by a limited amount of aircraft patrol in 
the mid-ocean gap, which proved to be extraordi- 
narily effective. A small number of VLR aircraft be- 
came available in March, and the USS Bogue [CVE] 
also sailed in support of the convoys. Considerable 
numbers of U-boats were sunk, but they continued 
to attack in force until early May when they were 
driven off from ONS 5 in the decisive convoy battle 
of the war. After May 17 no ships were sunk in the 
North Atlantic for some time. 

While aircraft were thus distinguishing themselves 
in the defense of convoys, the Coastal Command of- 
fensive against transit U-boats was also bearing fruit. 
S-band radar was introduced early in 1943, and the 
number of sightings and attacks soared. During May, 
37 U-boats were sunk in the Atlantic, 11 of them in 
the Bay of Biscay. The success of these operations 
continued until the end of the summer. 

By July the U-boats had dispersed to try to find a 
soft spot in Allied defenses. They failed completely, 
and found themselves attacked even in mid-ocean by 
CVE-based aircraft. In Atlantic waters a total of 34 
U-boats were sunk, mostly by aircraft. The result of 
such disastrous losses was a complete defeat of the 
U-boats, in which they gave up aggressive surfaced 
operations and submerged during daylight hours to 
avoid aircraft. Ultra-conservative tactics were em- 


ployed in crossing the Bay of Biscay. 

Having thus lost their mobility, the U-boats ac- 
complished nothing until September and October 
when they attempted to make a come-back against the 
North Atlantic convoys by employing acoustic hom- 
ing torpedoes against the escorts. They were beaten 
off with heavy losses— 22 U-boats sunk during Octo- 
ber in operations against convoys in order to sink 
three merchant vessels and one escort. An attempt to 
attack the convoys between the United Kingdom and 
Gibraltar was then made, but it was also frustrated 
and the U-boats forced into a completely defensive 
position. 

For the rest of the winter they adopted a policy of 
maximum submergence, designed to give them safety 
from Allied attack. Virtually no ships were sunk. 
The number of U-boats at sea declined markedly, 
and every effort was made to develop an effective 
search receiver for S-band radar to give them im- 
munity from detection by the Allies. 

It was not until the invasion of Normandy in June 

1944 that the U-boats showed any signs of aggressive 
action. Their effort to attack shipping in the English 
Channel was short-lived, however, as heavy air and 
surface patrols prevented them from achieving any 
success. They soon abandoned the surface in favor of 
Schnorchel operation which gave them virtual im- 
munity from detection by aircraft. By lying on the 
bottom the were able to utilize the poor sound condi- 
tions and frequent wrecks to gain considerable safety 
from Asdic. Operating in this way limited successes 
against shipping in British coastal waters were 
achieved during the summer, while sinkings of 
U-boats became less and less frequent. 

In August and September the U-boats withdrew 
from the Biscay bases to Norway, and then settled 
down to a small-scale offensive around Britain. They 
met with some success at first, but by the spring of 

1945 Allied surface craft had learned to deal with 
them under those conditions, and the Allied victories 
on land deprived them of bases and shore facilities. 
Their only chance for regaining the upper hand was 
the new high submerged speed U-boat, Type XXI, 
but due to production difficulties and the general 
German collapse none of them made an operational 
patrol against the Allies. 

With the German surrender in May 1945, U-boat 
warfare was to all intents and purposes ended. Jap- 
anese U-boats caused the Allies no serious concern 
during the remainder of World War II. 


TABLES AND CHARTS 


83 


Table 1. Average Monthly Shipping Losses and Construction of Allied and Neutral Nations. 
(By period and cause of loss and in thousands of gross tons.) 


Cause 

Periotl I 
Sept 39- 
June 40 

Period 11 
July 40- 
Mar 41 

Period 111 
Apr 41- 
Dec 41 

Period IV 
Jan 42- 
Sept 42 

Period V 
Oct 42- 
June 43 

Period M 
July 43- 
May 44 

• 

Period MI 
June 44- 
Apr 45 

World 
War 11 
Sept 39- 
Apr 45 

Sunk by U-boats 

106 

224 

175 

508 

394 

105 

57 

214 

“ aircraft 

29 

61 

76 

70 

21 

35 

8 

41 

“ ships 

14 

87 

17 

40 

7 

4 

2 

23 

“ mines 

58 

27 

20 

11 

9 

5 

15 

20 

other enemy 









action 

16 

5 

34 

26 

5 

2 

3 

12 

Total sunk by enemy 






— 

— 

— 

action 

223 

404 

322 

655 

436 

151 

85 

310 

Sunk by marine 









Casualty- 

58 

52 

40 

49 

55 

32 

39 

46 


— 

— 










Total losses— all causes 

281 

456 

362 

704 

491 

183 

124 

356 

New construction 

57 

114 

175 

515 

1026 

1160 

850 

580 


— 

— 

— 








Net monthly loss 

224 

342 

187 

189 





or gain 





535 

977 

726 

224 

Shipping available 









in million gross 









tons 

40.0 

37.8 

34.7 

33.0 

31.3 

36.1 

46.9 

55.0 


Table 2. 

Average number of U-boats sunk monthly 

— World- 

wide by periods and cause of sinking. 


Cause 

Period I 
Sept 39- 
June 40 

Period 11 
July 40- 
Mar 41 

Period III Period IV 
Apr 41- Jan 42- 

I)ec41 Sept 42 

Period V 
Oct 42- 
June 43 

Period VI 
July 43- 
May 44 

Period VII 
June 44- 
Apr 45 

World 
War II 
Sept 39- 
Apr 45 

Sunk by surface craft 

2.1 

1.7 

3.0 

3.6 

7.2 

7.5 

8.8 

5.0 

“ “ S/C & A/c: 

0.1 

0.3 

0.2 

0.9 

1.2 

2.1 

1.4 

0.9 

aircraft 

0.3 

0.2 

0.4 

2.2 

9.3 

11.3 

10.0 

5.1 

submarine 
other or unknou 

0.2 

n 

0.3 

0.4 

1.3 

1.4 

1.3 

1.5 

1.0 

causes 

0.5 

0.6 

0.7 

0.4 

0.8 

0.8 

2.6 

1.0 

Total sunk 

3.2 

3.1 

4.7 

8.4 

19.9 

23.0 

24.4 

13.0 


1'able 3. Approximate German U-boat position. 
(Ocean-going U-boats only — 500 tons or more.) 



Period 

At start 
of period 

Con- 

structed 

Sunk 

At end 
of period 

I 

Sept 39-June 40 

30 

15 

23 

22 

II 

July 40-Mar 41 

22 

45 

13 

54 

III 

Apr 41-Dec 41 

54 

174 

28 

200 

IV 

Jan 42-Sept 42 

200 

200 

50 

350 

V 

Oct 42-June 43 

350 

178 

142 

385 

VI 

July 43-May 44 

385 

250 

215 

400 

VII 

June 44-Apr 45 

400 

180 

234 

350 



84 


SUMMARY OF ANTISUBMARINE WARFARE, WORLD WAR II 




15 

AVERAGE LIFE 
OF U-BOAT AT 
SEA (in mos . ) 


M/V's SUNK 4 
PER U-BOAT 
MONTH AT 
SEA 0 




1 

— 

r\ c 




’ x'-x-xmsS' ' 




-crs^ 


104 


fOO 


AVERAGE 
NUMBER OF 
U-BOATS 
AT SEA 











TOTAL 

PERIOD: 

WORLD WAR I. 

PERIOD I. 

PERIOD II 

PERIOD III. 

PERIOD IV. 

PERIOD V. 

PERIOD VI. 

PERIOD VII 

WORLD WAR II 


Aug 1917 - 

Sep 1939 - 

Jul 1940 - 

Apr 1941- 

Jan 1942- 

Oct 1942- 

Jul 1943- 

Jvin 1944- 

Sep 1939 - 


Jul 1918 

Jun 1940 

Mar 1941 

Dec 1941 

Sep 1942 

Jun 1943 

May 1944 

Apr 1945 

Apr 1945 


(M/V ' s sunk 

U/B activity 

U/B's get 

Dispersion 

U/B's shift ' 

U/B's attack 

U/B's dis- 

U/B ' s use 



by U/ 3 ' s are 

in waters 

French bases 

of Allied 

to E. coast 

N. Atlantic 

perse, at- 

Schnorchel 



reduced in 

near England 


convoy 

of U.S. to 

convoys in 

tack suppos- 

to protect 



above table 


Operate in 

routes . 

attack M/V's 

"GAP". 

ed weak 

themselves 



to reflect 

Daylight at- 

Western Ap- 

U/B's start 


* 

spots in S. 

against A/C. 



smaller ton- 

tacks at 

proaches to 

Losses reach 

Large wolf 

Atlantic . 

U/B ' s oper- 



nage of 

periscope 

England . 

wolf pack 

peak in June 

packs over- 

U/B's stay 



M/V's in 

depth. 


attacks on 

1942; 140 

whelm es- 

ate mainly 



World War I) 


Night sur- 

convoys . 

ships sunk 

corts . 

on surface 

in "Invasion 




Allies use 

face at- 


by U/B's. 


and fight 

Area" near 



Allies used 

lightly es- 

tacks on 

U/B ' s move 

Extensive 

VLR A/C and 

back against 

England. 



convoys dur- 

corted con- 

convoys . 

westward and 

escort car- 

A/C. 




ing this 

voys . 


southward. 

convoying 

riers close 


U/B's use 



period. 


Allies in- 


along east 

"GAP". 

A/C inflict 

bottoming 



U/B ' s con- 

troduce me- 

Allies adopt 

coast in 


"heavy" los- 

tactics in 



Allies used 

centrate on 

ter radar. 

complete 

July 1942. 

S-band radar 

ses on U/B ' s 

shallow 



hydrophones 

independent 


trans-Atlan- 


used on A/C 

U/B's forced 

water . 



and depth 

shipping. 

3 leading 

tic escort. 

25-foot 

in Spring of 

U/B's lose 



charges . 

U/B's use 

U/B "aces" 


depth set- 

1943. 

to adopt 




lost in 

U.S. escorts 

ting on A/C 


maximum sub- 

French 'bases 

» 


A/C gener- 

electric 

March 1941 . 

sail with 

depth charg- 

A/C attacks 

mergence . 

move to Nor- 



ally inef- 

torpedo, (no 


convoys in 

es. 

much more 

U/B's lose 

way . 



fective 

wake) . 


September 


effective. 

U/B's still 



against U/B. 


1941. 

U/B ' s driv- 


offensive 




Allied S/C 



en away from 

U/B ' s badly 

power as a 

ineffective, 




use asdic 


Drop in ef- 

east coast 

beaten in 

result of 

due to lack 




successfully 


fectiveness 

of U.S. 

attacks on 

loss of mo- 

of mobility. 





of U/B ' s due 


convoy "ONS- 

bility . 

Afl-lied S/C 






to rapid ex- 


5 " in March 







pension of 


1943. 

U/B's use 

(using Squid 






U/B person- 



acoustic 

and Hedgehog 






nel . 


U/B's with- 

torpedo and 

in addition 








draw from N. 

GSR for S- 

to depth 






U.S. enters 
war in De- 
cember 1941 - 


Atlantic. 

band radar. 

charges) in- 
flict heavy 
losses on 
Schnorchel 
U/B's. 




M/V's SUNK 
BY U-BOATS 
PER MONTH 


U-BOATS SUNK 
PER MONTH 


M/V's SUNK 
PER 

U-BOAT SUNK 


Figure 1. U-boat and antisubmarine operations for the seven periods of World War II. 








TABLES AND CHARTS 


85 


8 3 TABLES AND CHARTS 

The outstanding statistical facts of the antisubma- 
rine war are summarized in the following tables and 
charts: 

1. Figure 1— presents figures measuring the magni- 
tude and effectiveness of U-boat and antisubma- 
rine operations for the seven periods of World 
War If. The “remarks” attached explain the out- 
standing characteristics of each period. 

2. Table 1 -presents Allied shipping losses due to 
various causes and gains through construction for 
each period. 


3. Figure 2-summarizes the information of Table 1 
in graphical form. 

4. Table 2-presents the average number of U-boats 
sunk monthly by cause for each period. 

5. Figure 3-summarizes the information of Table 2 
in graphical form. 

6. Table 3— presents German U-boat losses and 
gains through construction for each period. 

7. Table 4-presents total shipping losses and U- 
boat sinkings for each period. 

8. Table 5— gives the effectiveness of Allied attacks 
by aircraft and surface craft on U-boats in the At- 
lantic and Mediterranean for each period. 


Gross tons 



86 


SUMMARY OF ANTISUBMARINE WARFARE, WORLD WAR II 


26 

24 

22 

20 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 


CAUSE 


Other 


Submarine 


Aircraft 




PERI PERM PER III PER IV PER V PER VI PER VII WORLD WAR II 



SEP 39- JUL40- APR 41- JAN 42- OCT 42- JUL 43- JUN44 

JUN 40 MAR 41 DEC 41 SEP 42 JUN 43 MAY 4 4 APR 46 


SEP 39- 

APR 45 


Figure 3. Monthly U-boat sinkings by period and cause of sinking. 


Table 4. Shipping sunk by U-boat and U-boats sunk, by period (world- wide). 


Ship sunk by U-boat U-boats sunk 

Period Number 1000 gross tons German Italian Japanese Total 


I Sept 39-June 40 256 1,058 23 9 32 

II July 40-Mar 41 379 2,020 13 15 28 

III Apr 41-Dec 41 325 1,580 28 14 42 

IV Jan 42-Sept 42 878 4,575 50 15 11 76 

V Oct42-June43 603 3,546 142 16 18 180* 

VI July 43-Mar 44 192 1,150 215 10 27 252 

VII June44-Apr45 117 618 234 35 269 

VIII May45-Aug45 3 10 28 8 36 

Total -World Warn 2,753 14,557 733 79 99 915* 


Includes 4 Vichy French U-boats. 


TABLES AND CHARTS 


87 


Table 5. Quality of Allied attacks on C-boats. 
(Hy period — in Atlantic and Mediterranean.) 


Aircraft Surface Craft 

Percent resulting in at least Percent resulting in at least 

Some damage Sinking of U/B Some damage Sinking of U /B 


I 

Sept 39-Jnne 40 

10 

1 

Satisfactory data 

are 

II 

July 40-Mar 41 

10 

21/2 

not available for 1 

this 

III 

Apr 41-Dec 41 

25 

21/2 

early period. 


IV 

Jan 42-Sept 42 

20 

3 

15 

5 


Oct 42-June 43 

25 

10 

25 

10 

\’I 

July 43-May 44 

40 

25 

30 

20 

VII 

June 44-Apr 45 

35 

18 

35 

30 







PART II 


ANTISUBMARINE MEASURES 

F rom the historical summary of operations 
against submarines during World War 11 pre- 
sented in Part 1, many conclusions concerning the 
proper strategy and tactics of antisubmarine warfare 
[ASW] can be drawn. I he most important of these 
will be discussed in the following chapters and sub- 
stantiated by quantitative data from Operations Re- 
search Group studies. 

Most conclusions have to do with specific problems 
of tactics, for example, weapons for attacks, proper 
tactics for search, or methods of protecting convoys. 
These are all part of a general picture which involves 
the overall purpose of antisubmarine warfare and the 
various methods available for accomplishing that 
purpose. 

It is already evident from Part 1 that the aim of 
ASW is not simply the destruction of unfriendly sub- 
marines. If it were possible to sink all enemy sub- 
marines, the mission of ASW would, of course, be 
accomplished, but the forces available have never 
been sufficient to do this, for a number of reasons. 
The submarine is a small and elusive object in a large 
ocean and consequently very hard to find. When 
•found, it is a tough and inaccessible object to attack. 
As long as the enemy is able to build and launch sub- 
marines, he can keep some of them at sea, and even 
the highly effective Allied antisubmarine effort in 
World War II did not greatly diminish the size of the 
German U-boat fleet but merely checked its growth. 

Antisubmarine warfare must be thought of as a 
part of a complex overall military strategy whose final 
aim is to eliminate the enemy’s ability to wage war. 
This must ultimately be done by seizing or destroying 
the military or economic war machine— administra- 
tion, transportation, men, equipment, or production 
facilities— by striking at the heart of the enemy, not 
merely nibbling piecemeal at his periphery, or by 
convincing him of an ability to do so. When possible 
it is most efficient to ignore the periphery and proceed 
directly to the ultimate objective. This is the philoso- 
phy underlying the blitzkrieg and war of encircle- 
ment. 

The aim of ASW is to ensure the use of the oceans 
necessary for military operations intended to bring 
about the defeat of the enemy. It is an auxiliary oper- 


AND THEIR EFFECTIVENESS 

ation, necessary, but not sufficient, for overall success. 
In this sense, then, the actual aim of ASW is negative, 
to prevent enemy submarines from accomplishing 
their aim. 

The first step in this analysis of the subject is to 
outline the value of submarine operations to the 
enemy. The objective of ASW is to reduce this value 
to a minimum, and its general strategy is planned 
accordingly. 

SUBMARINE OPERATIONS 

The peculiar value of the submarine among naval 
craft is its ability to operate in enemy-held waters. 
Even when surfaced, a submarine is a small target for 
visual or radar detection. When submerged, it is com- 
pletely concealed except for detection by underwater 
sound, whose ranges are short and unreliable.--^ As a 
result, the submarine can operate in regions forbid- 
den to surface craft because of enemy patrol. In these 
regions aircraft operations are often impossible be- 
cause the regions are beyond the range of aircraft. 
Thus the submarine is the primary, and often the 
only, craft for carrying out operations at a long dis- 
tance from base in the areas of the enemy’s main 
strength. 

Numerous types of operation may be involved. 
The aim of some may be to gather information that 
can be gained only by an excursion into enemy terri- 
tory. In this class are routine scouting and weather- 
reporting missions and also those involving the land- 
ing of agents. Some are transport missions to supply 
isolated units which cannot be reached by other 
means. The main mission of the submarine, however, 
is offensive— to attack the enemy’s ships, both com- 
batant and merchant. Past experience has shown that 
the most valuable submarine activity has been that of 
attacking merchant shipping, and antisubmarine 
warfare is of urgent concern on this account. 

Man being a terrestrial animal, the oceans are valu- 
able to him only as a means of transportation from 
one piece of land to another or as a barrier between 

a Visual detection of a submerged submarine is possible only 
in very rare cases. Magnetic detection is effective only at very 
short ranges, much shorter than those of underwater sound. 


89 


90 


ANTISUBMARINE PRINCIPLES 


them. Control of the ocean secures this use to the con- 
troller and denies it to his enemy. While an antiship- 
ping offensive by submarine does not give control of 
the ocean, it serves to deny free use to the enemy, and 
in this negative sense the submarine force may accom- 
plish quite complete control. To the extent that this 
is done and the enemy prevented or hindered from 
transporting necessary cargoes, the submarine offen- 
sive makes a major contribution to the progress of the 
war. 

In the submarine’s antishipping offensive three 
things must be accomplished: achieving contacts on 
ships, approach to within torpedo range, and final 
attack. The first of these is essentially a search prob- 
lem of the kind discussed in Volume 2B, Search and 
Screening, with submarine as searching craft. Detec- 
tion may be made by visual, radar, or sonar means. 
Visual detection has had the largest range and sonar 
the shortest, as a rule, though the ranges depend on 
conditions. Visual detection, for instance, will be 
ineffective on a dark night, and sonar ineffective in 
poor sound conditions. 

Since the submarine’s speed is low, it is not a very 
efficient searching craft and must operate in regions 
of high shipping density to make a large number of 
contacts. To select the high-density areas a knowledge 
of the expected positions of enemy ships is required, 
which may be based on intelligence information or 
gained locally by coordination between submarines. 
The “wolf-pack” tactics of German U-boats accom- 
plished this by homing many submarines to each con- 
tact. Once a single U-boat made contact, the informa- 
tion was used to permit others to achieve contact as 
well. 

When contact has been made, the submarine must 
approach to within torpedo range before an attack 
can be launched. If the submarine's speed is greater 
than that of the target, the approach is not very diffi- 
cult, and practically all ships contacted may be at- 
tacked. However, if the submarine is forced to oper- 
ate submerged (or if the target is very fast), approach 
is only possible from positions ahead of the target, 
and slight errors in the approach may result in failure 
to achieve the proper position. 

Once firing position is reached, torpedoes are fired. 
Their chance of success depends on the accuracy of 
the torpedo fire and on the physical characteristics 
of the torpedo used. 

The primary aim of ASW is to reduce the effective- 
ness with which the submarines carry out these steps, 


and the success of an antisubmarine effort is to be 
assessed in these terms. Antisubmarine warfare is not 
an end in itself, but merely a means of ensuring that 
the ability to use the ocean for transportation is main- 
tained at the best level possible. 

ANTISUBMARINE MEASURES 

Some antisubmarine measures are specifically in- 
tended to hinder the submarine in carrying out a par- 
ticular phase of its antishipping operations. Evasive 
routing of convoys to avoid known submarine posi- 
tions is useful solely in reducing the submarine’s 
chance of contacting the convoys. If all the ships 
could always dodge the submarines, the latters’ effec- 
tiveness would be much reduced. Such a procedure 
would slow the ships down appreciably, however, so 
that the submarine effort would not be entirely 
wasted. On the other hand, most measures designed 
to combat submarines serve a multiple purpose. 
Maintaining aircraft patrol in the vicinity of a con- 
voy not only may lead to sightings of submarines, 
thus permitting evasion by the convoy, but these air- 
craft also force submarines to dive, thus hindering 
them in their efforts to track a convoy and approach 
it. Finally, an occasional aircraft may be fortunate 
enough to attack a submarine and sink it. The over- 
all value of any measure must be based on all the 
ways in which it serves to frustrate the submarine. 

Most obvious among the measures designed to re- 
duce the submarine’s rate of contacting ships is eva- 
sive routing. If the submarine positions are fairly 
well known, they can be avoided and the density of 
shipping in their vicinity greatly reduced. Convoying 
is another measure which, in effect, reduces the sub- 
marine contact rate. If there are n ships in each con- 
voy, on the average, the submarine contacts only 
about 1 jn as many convoys as it would ships. Since n 
may be made as large as 50 to 100, the resulting gain 
is considerable. Patrol by aircraft also serves to reduce 
the contact rate, as indicated earlier, by forcing the 
submarine to spend a large fraction of its time sub- 
merged. This reduces its detection range and inter- 
feres with the formation of wolf packs. In a less direct 
way, both surface and aircraft patrols and hunter- 
killer operations have the same effect. A concentra- 
tion of offensive operations in areas of high density of 
shipping serves to force the submarines out of these 
areas and to make them operate where their chances 
of contacting ships are not so good. The apparent aim 


ANTISUBMARINE MEASURES 


91 


of such an offensive is to sink U-boats, but its chief 
effect may be to force them to adopt less profitable 
tactics, and this effect continues to be valuable even 
if sinkings of U-boats be reduced to a negligible level. 

The methods of hindering submarine approach 
consist of various forms of escort, both for convoys 
and single ships. Aerial escort tends to force the sub- 
marines to submerge and thus restricts their mobility, 
making it impossible for them to trail the convoy or 
to overtake it from the flanks or rear and reach a posi- 
tion ahead from which attack can be made. Even if 
the submarine is ahead of the convoy when she con- 
tacts it, a submerged approach is more difficult than 
one carried out partially on the surface and is more 
likely to result in errors. 

If the submarine boldly elects to stay on the surface 
for approach to the convoy, it is very likely that it will 
be detected and attacked in the process, which ex- 
perience is almost certain to eliminate any possibility 
of successful offensive action on its part. Correspond- 
ingly, surface craft escorts interfere with the later 
stages of the approach. Having to try to avoid them 
complicates the submarine's problem, but, if it fails 
to do so, it is likely to be detected and counterattacked 
before reaching torpedo-firing position. The sub- 
marine’s problem can be made still more difficult by 
use of the highest possible speed of ship and by zigzag, 
when possible. 

Attempts to interfere with the submarine’s final 
attack, after it has reached firing position, have not 
been very successful in the past. To some extent the 
mere presence of surface escorts may constitute such 
interference. Zigzagging reduces the accuracy of tor- 
pedo fire somewhat. A maneuverable ship may turn 
to avoid the torpedo if the ship has sufficient warning, 
and special torpedo detectors may be devised to aid 
in doing so. Antitorpedo nets and other devices can 
be streamed to intercept the torpedo. All these are 
examples of devices and tactics designed to reduce the 
effectiveness of the submarine torpedo fire. 

Supplementing these essentially defensive methods 
of interfering with the submarine’s accomplishment 
of its objective is the offensive phase of ASW. The 
most certain way of preventing a submarine from 
sinking ships is to sink it first. In this sense, sinking a 


submarine is equivalent to saving as many ships as it 
would normally sink during the remainder of its 
operational lifetime. If, for example, the average 
submarine makes a total of ten operational patrols 
and sinks two ships in each, sinking the submarine 
saves an average of ten ships, because the submarine 
is likely to be about half through its normal life when 
sunk.^ In addition to this direct diminution of the 
submarine fleet, the sinkings tend to reduce the state 
of training and experience of the submarine crews. 
If, for example, the rate of sinking can be kept high 
enough to give the submarine (or crew member) an 
expected life at sea of only four patrols, there will be 
very few men available with the experience of, say, 
ten patrols. The resulting dearth of experienced per- 
sonnel is a handicap very difficult to assess, but un- 
questionably of considerable practical importance. 
Only a fraction of the value of offensive operations 
can be represented by the numerical decrement 
achieved in the enemy’s submarine fleet. 

There is normally an appreciable effect upon 
morale as well. The effectiveness of submarine opera- 
tions cannot be divorced from the skill and deter- 
mination of the submarine force’s personnel. By 
selectively eliminating experienced men, the high 
rate of loss reduces not only their overall skill but also 
their determination. The submarine’s chance of be- 
ing sunk if it endeavors to attack a certain convoy, for 
example, cannot but bear a greater importance to its 
crew than to the theoretical strategist. The submarine 
crews will accordingly avoid operations which in- 
volve high losses to themselves, even though theoreti- 
cally profitable, unless their psychology includes a 
definite suicidal tendency. 

From this brief outline it is evident that ASW in- 
volves a great many different aspects. The following 
chapters will discuss a number of specific problems in 
order to illustrate the general principles involved. 
The subjects for discussion have been chosen on a 
dual basis: first, for their importance in antisub- 
marine strategy and tactics, and, second, for their 
interest as examples of the methods of Operations 
Research. 


b This type of comparison can be mdde most clearly on the 
basis of sinking rates, as in Chapter 13. 






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Chapter 9 

SAFETY OF INDEPENDENT SHIPPING 


AS WAS pointed out in Part I (page 2), the sub- 
xjL marine’s problem is one of first contacting a ship, 
then approaching to a good torpedo-firing position, 
and finally securing a torpedo hit. It is appropriate to 
break down an analysis of measures designed to in- 
crease the safety of shipping into the same general 
categories. 

9 » REDUCTION IN SUBMARINE’S 
ABILITY TO CONTACT SHIPS 

In the first place, anything which can be done to 
reduce the number of ships contacted by each sub- 
marine will increase the safety of the ships. Consider, 
for example, a ship making a trip between two points 
through an ocean infested with submarines. 

There are D submarines per sq mile in the ocean, 
each of which has a sweep rate of Q sq miles per 
hour.* The length of the ship’s track is / miles and its 
speed V. Then the expected number of times that sub- 
marines will contact the ship is 

No = DqI ( 1 ) 

To make A^o small, D, Q, or Ij v must be decreased. 
These factors will be considered in turn. 


Reduction in Submarine Density 

One method of reducing the submarine density to 
which the ship is exposed is to avoid regions in which 
submarines are concentrated. If it is possible to 
choose between a route along which there are many 
submarines and one where there are few, the latter 
is certainly the better. When a fairly reliable estimate 
of submarine positions is available, much can be 
done by evasive routing to reduce contacts, but such 
information is by no means always available. If this 
information is not available, a wide dispersion of 
shipping routes can be employed, forcing the enemy 
to deploy his forces in the same way, instead of per- 



Figure 1. Effect of aircraft flying on the distribution of 
U-boats in Gibraltar-lVforocco area. 


mitting him to concentrate them against a well-de- 
fined shipping lane. 

In an indirect way, offensive antisubmarine opera- 
tions serve to reduce D, first, by sinking submarines 
and thus reducing their number at sea, and, second, 
by preventing them from concentrating in the most 
profitable areas. Aircraft patrols frequently can be 
used to force U-boats out of the regions of the highest 
concentration of ships. As an example of this process, 
a graph of U-boat density in the Gibraltar-Morocco 
region is shown in Figure 1. 

Immediately after the Allied landings in North 
Africa, the U-boats were concentrated close to shore 
in the region of highest density of merchant ships. 
Almost immediately, however, aircraft patrol forced 
them to give up the inshore concentration, which 
had, no doubt, resulted from a special effort to break 
up the invasion. The effect of this change in U-boat 
distribution on their expected number of contacts 
on ships can be estimated very simply. In any particu- 
lar region, the expected number of contacts is 

A^ = I^v/B X A X X (2) 


a The sweep rate Q is defined in Volume 2B, Search and 
Screening. Q gives the effective area which the searcher is able 
to inspect completely in a unit of time. 


where A = area of region, Djj/b = density of U-boats, 
and Dji^/y = density of merchant vessels. This ex- 
pected number must be calculated separately for 


93 


94 


SAFETY OF INDEPENDENT SHIPPING 




LATERAL RANGE IN MILES 


Figure 2. Lateral range curves, submarine detection of 
merchant ships. 


each region and summed. If this is done and a value 
for Q of 100 sq miles per hour is taken as typical of a 
submarine search rate for merchant ships, then we 
would expect about 60 contacts per month by U- 
boats on merchant ships before flying had forced 
them out, and 30 contacts per month afterwards. It 
would be reasonable to conclude that the flying in- 
volved'’ had cut the danger to ships approximately in 
half. 


Reduction in Submarine Sweep Rate 


curves gives s the sweep width, the values being as 
given in Table 1. 

Table 1. Submarine sweep width in searching for mer- 
chant ships. 



Day 

Night 

Average 

Surfaced (miles) 

14.5 

9.4 

12 

Submerged (miles) 

9.8 

4.2 

7 


The overall effect of forcing the submarine to sub- 
merge appears to be a reduction of 40 per cent or so 
in the sweep width. We can, then, consider half the 
sweep width to be the effective range of contact, 
which would be 6 miles surfaced and 3.5 miles sub- 
merged. If submarines remained hxed in the ocean, 
there would be a proportional reduction in the num- 
ber of ships contacted by the submarine, but since 
the surfaced submarine is able to add something to 
its search capabilities by patrolling at about 10 knots, 
the speed effect must also be taken into account, 
though it proves to be rather small. 

This speed effect is treated in Chapter 1 of Volume 
2B, Search and Screening. The result is expressed in 
equation (5) of that chapter, which allows us to write 
the search rate Q of equation (1) explicitly as 


To some extent, aircraft patrol may fail to drive 
U-boats out of an area as described above and merely 
force them to spend most of their time submerged. 
This also serves to make ships safer because it re- 
duces the submarine’s sweep rate. The primary rea- 
son for the reduction is the decrease in the range of 
detection, though the drop in submarine speed also 
has some effect. The submerged submarine is rela- 
tively blind compared with a surfaced one, since 
visual detection must be through periscope and also 
since radar cannot be used very conveniently. 

Operational data on the magnitude of these effects 
can be derived from the first contact ranges reported 
by U. S. submarines. From these data the lateral 
range curves of Figure 2 are plotted. The assumption 
has been made that there is a definite range at which 
contact will be made for each time that a submarine 
meets a ship, though these ranges vary widely from 
one occasion to the next.'^ The area under these 

b Total flying hours were: November, 933; December, 1833: 
January, 2167; February, 2120. 

c See Chapter 2 of \^olume 2B, Search a)i(l Screening, for a 
complete e.xposition of contact phenomena. 


Q = ^ / V 1 — sin- 8 sin- ip dip; 

sinS=^y^, (S) 

n -t V ^ ^ 

where 

() = sweep rate, 

A^o = expected number of contacts per unit time, 

N = number of ships per square mile, 

R = effective range of contact, 
n = submarine speed, 

V = merchant vessel speed. 

From equation (3) we can calculate Q, using tables 
to determine values of the elliptic integral involved. 
With the assumption that the merchant ships in- 
volved make 10 knots, the surfaced submarine 10 
knots, and the submerged submarine 3 knots, the re- 
sults are as presented in Table 2. 

The net conclusion is, therefore, that a submerged 
submarine has been able to contact only about half 
as many ships as a surfaced submarine and that anti- 
submarine patrol which forces submarines to sub- 



REDUCTION IN SUBMARINE’S ABILITY TO APPROACH SHIPS 


95 


"I'ablk 2. Sweep rale otsubnuuiiie in conlacting merchant 
ships. 



Submarine 

Range of 



.speed 

Contact 

Sweep rate 


(u) 

(R) 

(Q) 

Surfaced 

lOkt 

6 miles 

150 sq mi/hr 

Submerged 

3kt 

3.5 miles 

70 sq mi/hr 


merge most of the time has an appreciable effect in 
reducing contacts. 

Another line along which effort might reasonably 
be expended to reduce the submarine’s contact rate 
is camouflage. Since, however, it is by no means easy 
to devise a camouflage which is effective in this sense 
under all the varied conditions met with in practice, 
relatively little has been done, and operational data 
on the subject are not available. 

^ Reduction in Ship’s Time at Sea 

The factor l/v in equation (1) is simply the time 
the ship spends in the dangerous region. To keep this 
time small, the track length / should be made as short 
as possible and high ship speed used. The advantages 
of high ship speed in this connection are offset, how- 
ever, by an increase in Q with ship speed. The most 
important consideration is length of ship’s track in 
dangerous waters. 

92 REDUCTION IN SUBMARINE’S 

ABILITY TO APPROACH SHIPS 

When the submarine has made a contact, it must 
still make an approach to torpedo-firing position 
before launching an attack, and it is by no means 
certain that it will be able to do so. In a typical 
period only 41 per cent of the independent merchant 
vessels contacted by United States submarines were 



Figure 3. Ships sunk per million miles of track in a 
U-boat density of 1 per million square miles, as a func- 
tion of ship speed. 


attacked. Some of the remaining 59 per cent were not 
worth attacking, but many were not attacked because 
the submarine was unable to close to the desired 
position for launching a torpedo. The quantitative 
importance of the approach problem is made more 
evident by an analysis of the effect of merchant 
vessel speed on the safety of independent ships, based 
on Allied losses to German U-boats. 

9.2.1 The Effect of Ship Speed on 
Submarine Approach 

In order to study the effect of speed, it is necessary 
to assign the ships to speed classes and then deter- 
mine the number of ships sunk in each class and the 
overall exposure of each class to submarines. This 
overall exposure must take into account both the 
total distance traveled by ships of each class and the 
density of U-boats along the ships’ courses, as indi- 
cated by equation (1). 

In order to do this a study was made in which the 
Atlantic was separated into areas. In each area the 
mileage traveled by independents of different speed 
classes and the average density of U-boats were taken 
month by month. A ship which sails 2000 miles in 
waters with one U-boat per million sq miles is ex- 
posed to about the same risk as one traveling 1000 
miles in waters with two U-boats per million sq miles. 
Hence, the exposure for each speed class is given by 
the product of the mileage and the U-boat density. 
This was done for seven speed classes, covering a 
period of 4 months and four areas. 

The number of casualties per unit of exposure is 
a measure of the safety of the speed class in question, 
and this is plotted in Figure 3 as a function of speed. 
The sharp decrease in sinkings at speeds between 10 
and 15 knots is very obvious. In order to explain this 
result the following analysis of submarine approach 
methods is required. 

If a ship is sighted in a favorable position, the 
U-boat may approach either surfaced or submerged 
and attack directly. If the U-boat is not in a favorable 
position, it has to estimate course and speed and fol- 
low the ship until it can close the range or get into 
position ahead for a submerged approach. The direct 
attack may be called Method A, the attack following 
a chase. Method B. Since Method B provides a good 
opportunity for tracking and obtaining torpedo-fir- 
ing data, it is frequently used, a typical procedure 
being to track until dark and then attack. 


96 


SAFETY OF INDEPENDENT SHIPPING 


Method B requires that the U-boat be able to over- 
take the ship, namely, that the ship speed be not 
greater than about 15 knots (14 knots appears to be 
the critical speed in Figure 3). Method A, on the 
other hand, may be used against ships of any speed, 
though the number of times the U-boat is in favor- 
able position will decrease as the speed of the ship 
increases. This decrease certainly does not account 
for the sharp rise in the curve of Figure 3, which is 
due to the advent of attacks made by Method B. A 
diagram is helpful in making clear the significance 
of these two methods. 

In Figure 4, the submerged approach zone and 
effective contact range are plotted. The shaded area 
drawn to 2000 yd on either side of the ship is taken 
as a rough approximation to the zone in which a 
submarine has a good chance of scoring a torpedo hit. 
Suppose that a submarine which makes contact while 
ahead of the ship in the submerged approach zone 
can make an attack by Method A with average 
chance of success k. Then the crosshatched line gives 
the sweep width for Method A attacks. For a very fast 
ship, the sweep width is approximately 2 miles, and 
increases to about 12 miles for very slow ships as 
the limiting approach angle increases. Then the ex- 
pected number of Method A attacks per million 
miles of track in a U-boat density of one per mil- 
lion square miles is given by the length of this line 
and the expected number of sinkings is k times as 
great. 

Assuming that submarine speed (under water) = 
5 knots and that k = 0.20, the dotted line in Figure 
5 is a rough graph of expected sinkings by Method A. 
For ships of speeds of 15 knots or greater, all observed 
sinkings are accounted for, but many additional 
ships of speeds less than 15 knots were sunk, presum- 
ably by Method B. Either the submarine is enabled 




0 5 10 15 20 25 


SHIP'S SPEED IN KNOTS 

Figure 5. Ships sunk per million miles of track in a 

U-boat density of 1 per million sq miles. 

to track the ship and wait for a very favorable oppor- 
tunity to attack or is able to try again after a failure, 
in order to attain a high eventual level of success. 

For purposes of comparison, calculate the number 
of ships that would be sunk if every ship contacted by 
U-boats were sunk, using equation (1). If we take the 
example previously discussed, a 10-knot ship and 
assume that the U-boats are surfaced, then conditions 
corresponding to Figure 5 are 
D = 1/1,000,000, 

I = 1 , 000 , 000 , 

10 , 

150. 

We have 

1/1,000,000 X 150 X 1,000,000/10 
= 15 ships contacted per million miles of track. 

dims we woidd expect 15 ships to be contacted, 
whereas Figure 5 shows 10 sunk under these condi- 
tions. This seems like a rather high percentage, but 
it should be remembered that the conditions are on 
the whole rather favorable to the U-boat — a slow 
ship, unescorted, with little or no air patrol. A faster 
ship would be very considerably safer, though the 
number of times it is contacted per mile of track 
would not be greatly different. 

To summarize, then, there are three important 
speed classes. 

1. High-speed ships — sufficiently fast that the sub- 
marine cannot track or overtake them. The sinkings 
are low and not greatly dependent upon speed, 
though the additional speed of the ship clearly makes 
the submarine’s problem increasingly difficult. 

2. Low-speed ships — so slow that the submarine 
can track and overtake without difficulty. Sink- 


REDUCTION IN SUBMARINE’S ABILITY TO APPROACH SHIPS 


97 


I ablk 3. ItKicpendenl inerchaiu vessel losses, Caribbean Sea Frontier 
(July 1942-Feb 1943). 




.Average 




.Average 

No. of 


U-boat 

Month 

No. of 

independent 

Independent 

sinking 

U-boats 

M/V 

M/V sunk 

rate 

July 1942 

5.1 

11.7 

16 

210 

Aug 

6.4 

12.6 

6 

60 i 

Sept 

9.0 

11.7 

20 

'so }‘*® 

Oct 

8.6 

11.4 

9 

\ov 

5.1 

13.6 

9 

{'00 

Dec 

4.0 

11.1 

5 

Jan 1943 

3.0 

11.1 

0 

0 

Feb 

1.8 

10.0 

0 


Flying 
hr from 
Trinidad 


2.400 
5,000 
5,000 
4,700 

4.400 
5,100 
5,000 


ings are approximately ten times as great as in the 
high speed case and are not critically dependent on 
speed. 

3. Intermediate-speed ships — for which there is 
an abrupt transition from the conditions of class 2 
to class 1 and whose losses depend strongly on speed. 

The critical speed is one approximately equal to 
the surfaced speed of the submarine. A change in sub- 
marine speed would result in a corresponding change 
in the curves of Figures 4 and 5. With the U-boat 
capabilities of World Wir II, the critical importance 
of speed in the 10- to 14-knot range is obvious. Any 
measure, such as frequent dry-docking to clean the 
bottom, which might give an extra knot or two of 
speed in this range would be well worth while for a 
ship destined to sail independently through subma- 
rine waters. 

^22 The Effect of Aircraft Patrol 
on Submarine Approach 

1 he important role played by surfaced pursuit and 
tracking on the part of the submarine makes it evi- 
dent that any measure which keeps the submarine 
submerged a considerable part of the time will 
greatly reduce its chance of converting a contact into 
a sinking. Consequently, aircraft patrol, which tends 
to force the submarine down, might be expected to 
eliminate Method B attacks and greatly increase the 
safety of independent ships. The reduction in sink- 
ing rate indicated for 10-knot ships by Figure 5 is 83 
per cent; that is, about one-sixth as many ships would 
be lost as when surfaced tracking was possible. 

Operational data showing a clear comparison be- 
tween surfaced and submerged operation are diffi- 
cult to obtain without great differences in time and 


place. In addition, the advent of aircraft flying is 
normally accompanied by the start of convoying, and 
the effects of each are difficult to distinguish. For 
example, the start of convoying would normally in- 
crease the average speed of independent ships since 
the slow ships would be put in convoy. As a result, 
independent ships would appear to become safer. 
The type of result which may be obtained is shown in 
Table 3, which presents data from the Caribbean 
Sea Frontier during the fall of 1942. The area con- 
sidered includes the first 120 miles from shore in the 
Caribbean Sea Frontier— West and involves primarily 
action in the vicinity of Trinidad. As a rough meas- 
ure of the aircraft flying involved, data on flying in 
the Trinidad sector are used. The U-boat sinking 
rate (in sq miles per hour) is given by the following 
equation. 

Sinking rate = 

(M/V sunk) (Area involved) 

(M/V’s in area) x (U/B’s in area) x (hr/mo) 

( 4 ) 

In this case the area involved is about 600,000 sq 
miles. 

From a consideration of the first and last 2-month 
periods, it is apparent that an increase in flying has 
taken place, accompanied by a decrease in the sink- 
ing rate. But it is not evident whether the former was 
indeed the cause of the latter, and, if so, whether the 
Method A and Method B considerations involved 
had anything to do with it. When the sinking rate 
and flying are plotted graphically as in Figure 6, it 
appears that there was a lag of several months be- 
tween the rise in the flying curve and the decrease in 
sinking rate. At the time the increase in flying was 


98 


SAFETY OF INDEPENDENT SHIPPING 



yiUL AUG SEP OCT NOV DEC JAN FEB MAR 
1942 19^3 

Figure 6. Sinking rate and flying, by months (Caribbean 
Sea Frontier, West). 

made, the U-boat sinking rate dropped only slightly. 
This suggests that the flying did not cause the drop 
in sinking rate directly, but that the increased anti- 
submarine effort eventually (about January 1943) 
caused the U-boats to abandon their offensive in that 
area and adopt more conservative tactics. As is evi- 
dent from Table 3, they actually started to withdraw 
from the area in November and were practically all 
gone by February. The drop in sinking rate probably 
means that the few remaining made much less of an 
effort to attack ships than they had done some 
months before, rather than that aircraft flying di- 
rectly prevented them from attacking by keeping 
them submerged. 

The flying was not, in fact, sufficient to do so, as 
can be seen from an elementary calculation in which 
we estimate the amount of flying that would be re- 
cjuired to prevent surfaced tracking by submarines. 

If for example, it is assumed that to deny a sub- 
marine surfaced operation it must see a plane and be 
forced to dive at least twice a day, the number of fly- 
ing hours required to produce this effect can be cal- 
culated. A sort of sweep rate can be assigned to the 
aircraft by supposing that all submarines within 5 
miles of the track are forced to dive, and no others 
(probably a rather generous figure). Then the air- 
craft must cover the area involved 60 times during 
the month, a total of 60 x 600,000 = 36,000,000 sq 
miles covered. But the aircraft’s sweep rate is 2 x 5 x 
125 sq miles per hour, assuming an aircraft speed of 
125 knots. Hence, the total number of flying hours 
required is 

Flying hours = ^ = 29,000 per month. 

A X a X 1 

The actual flying recorded is not, strictly, the total 
amount in the area under consideration, but the 
total from Trinidad. Some of it was outside the area, 
and some other flying was done in the area. Never- 


theless, the total flying in the area considered was 
certainly a good deal less than 29,000 hours per 
month and could not be expected to eliminate sur- 
faced operation of U-boats. 

Sufficient flying to eliminate surfaced operation 
can ordinarily be accomplished only in a very limited 
area. In the vicinity of the British Isles, however, 
such a situation existed during the last months of 
World War II. U-boats operated totally submerged, 
using Schnorchel in the most heavily patrolled in- 
shore waters. In view of the densities of U-boats and 
ships involved, however, they did not make very 
many sinkings. Many factors besides aircraft patrol 
were important, and a detailed analysis of the situa- 
tion would be extremely complicated. It is men- 
tioned merely as evidence that submarines may at 
times go to the extreme of totally submerged 
operation. 

9.2.3 Effect of Zigzag on Submarine 
Approach 

With respect to the submarine’s approach prob- 
lem the importance of high ship speed and of air- 
craft patrol to enforce submerged operation have 
been discussed. Both of these are effective antisubma- 
rine measures in that they make it difficult for the 
submarine to carry out an approach to the ship. 
Another measure with the same aim is worth consid- 
ering: the zigzag. By making fairly radical turns at 
irregular intervals, the ships can make it more diffi- 
cult for the submarine to approach to a good firing 
position and to secure good torpedo-firing data. The 
net result is that the submarine will tend to be forced 
to fire from a poorer position with poorer accuracy 
and will therefore have less chance of securing a hit. 
At present, however, quantitative data on the effect 
of zigzagging are not available. 

93 REDUCTION IN EFFECTIVENESS 
OF TORPEDO FIRE 

Once the submarine has reached position and 
fired its torpedoes, there are still certain defensive 
measures which can be taken to reduce sinkings. Tor- 
pedo detectors have been dvised to warn of the tor- 
pedo’s approach, thus permitting a turn to avoid it, 
but they have not been effective on merchant ships. 
Ships should be built so as to be as likely as possible 
to stay afloat when hit. Special protective devices 


SUMMARY 


99 


such as antitorpedo nets may be employed. Because 
of their extensive use, operational data on their effec- 
tiveness are of considerable interest. 

To January 1, 1944, 25 ships fitted with Admiralty 
Net Defense [AND] nets were reported to have been 
torpedoed, with the results given in Table 4. 


Table 4. Results of torpedoes fired at ships fitted with 
anti torpedo nets. 



Sunk 

Damaged 

Undamaged 

Nets not in use when 
attacked 

9 

3 

0 

Nets in use 

4 

3 

3 

Use of nets unknown 

3 

0 

0 


Of the ships not using nets, 75 per cent of those 
fired at were sunk, but with nets streamed only 40 
per cent fired at were lost. The chance of being sunk 
is still about half what it would be without nets, be- 
cause some torpedoes either go through the nets or 
hit at unprotected ends of the ship. Table 4 above 
also indicates that about half the torpedoings oc- 
curred when nets were not streamed, so that the ulti- 
mate effect is that ships fitted with nets suffer about 
three-quarters the losses that they would sustain 
without the nets. 

These are the overall results, of course, and depend 
on the manner in which the nets are used. In the 


period considered, many of the ships were in convoy 
and streamed nets only when attack was considered 
likely. Bad weather is also a cause for nonstreaming 
of nets. This factor enters into the overall usefulness 
of nets. 

A serious drawback of net defense is the attendant 
reduction in speed when nets are streamed. This 
amounts to about 17 per cent. For a 14-knot ship, the 
reduction in speed to 11 1/2 knots increases the danger 
to the ship by a factor of about 31 / 2 , if the ship is sail- 
ing independently, according to Figure 3. This in- 
crease more than overbalances the 50 per cent reduc- 
tion afforded by the nets; consequently, their use 
would not be desirable in this case. If, however, the 
original speed were only 10 knots, reduction to 8 
knots would not seriously increase the danger, and 
nets would be most desirable for use in submarine 
waters. 

SUMMARY 

To summarize, then, there are a number of de- 
fensive measures which may be taken to reduce sink- 
ings of independent ships. Most of them are effective 
because they make it more difficult for the submarine 
to carry out its approach and attack. The overall ef- 
fectiveness of these measures is limited, however, and 
the most successful defensive measure has been tl ^ 
use of escorted convoys, which will now be discussed 


Chapter 10 

CONVOYING AND ESCORT OF SHIPPING 


T he primary advantage of a convoy system is the 
concentration of defense that it allows. Since it is 
impossible to provide an escort for every individual 
ship at sea, ships must sail in groups so that each 
group may be adequately escorted. A secondary ad- 
vantage is the reduction in number of units at sea for 
the submarine to contact, since the convoy becomes 
the unit instead of the individual ship. It is con- 
venient to analyze the gain achieved from convoying 
in the same three steps as were discussed in the last 
chapter: contacts made by the enemy, his ability to 
make an approach, and his chance of sinking ships 
once firing position is attained. 

10.1 the gain in safety 

BY CONVOYING 

10.1.1 Reduction in Number of Contacts 
Made by Submarines 

The institution of the convoy system results in a 
considerable reduction in the number of contacts 
made by submarines. The number of sightings made 
depends on the number of units present to be sighted. 
Clearly, 100 independent ships sailing in a given area 
represent 100 opportunities for sighting, while the 



Figure 1. Submarine sweep width on convoy. 


same number of ships in two convoys of 50 ships each 
offer only two opportunities for sighting. Each sight- 
ing, however, gives 50 targets and is accordingly of in- 
creased value, as will appear from subsequent sec- 
tions. This effect tends to neutralize the benefit 
derived from grouping, but only to a small extent. 
In addition, an increased range of detection tends to 
increase the number of convoys that are contacted. 
Although the range of detection on a 50-ship convoy 
is by no means 50 times that on a single ship, it is 
appreciably greater, and some consideration of the 
increase is necessary. 

Visual sightings, if made on masts or superstruc- 
ture, will be made at about the same range on con- 
voys as on independent vessels. A large convoy covers 
a sufficiently large area to increase the sweep rate 
about 50 per cent, as shown in Figure 1, but this is 
not a serious increase. Submarines often sight ships 
by smoke, however, which may be seen at distances 
up to 40 miles. A single ship may make smoke only 
a small fraction of the time, but usually there is at 
least one ship in a convoy which is smoking, and 
smoke becomes very much more important. If h is 
the fraction of time that each ship smokes, the 
range on a ship itself, R 2 the range on the smoke, and 
n the number of ships in convoy, then 

Fraction of time convoy smokes = 1 — (1 — by 

Average range on convoy 

= ( 1 . - byR^ + [1 - (1 - 

= R.,-(\-bY{R.-R,). ( 1 ) 

For convoys of as many as 50 ships, (1 — 5)” is so small 
that the sighting range is approximately R 2 , the 
range of detecting smoke. For a plausible set of values 
(i?i == 4 miles; R 2 = 24 miles; b = 0.10), equation 
(1) gives the following average ranges. 

Single ship 6 miles 

8-ship convoy 1 1 miles 

64-ship convoy 23 miles 

The submarine’s radar will generally detect a con- 
voy at a longer range than a single ship, because the 
convoy presents a larger reflecting target. Since, how- 


100 


THE GAIN IN SAFETY BY CONVOYING 


101 


ever, the intensity of the radar echo decreases very 
rapidly with increasing range to the target (normally 
as 1/r^ or faster), the increase in range is not very 
great. Operational data giving ranges of submarine 
radar on large convoys are not available. U. S. sub- 
marines have obtained ranges about 35 per cent 
longer on convoys of about 3 to 4 ships than on single 
ships. Aircraft radar ranges on large Atlantic convoys 
have been only 10 to 25 per cent greater than on 
single ships. It would not be expected, therefore, 
that grouping of ships in convoy would cause a seri- 
ous increase in the range of radar detection. 

The range of sonar detection will be appreciably 
increased. The intensity of sound produced by a con- 
voy of n ships is about n times that of a single ship. 
On this basis it has been estimated that the range on 
a oO-ship convoy under favorable listening conditions 
would be about three times that on a single ship. 
Under less favorable conditions the ratio would be 
smaller. 

In order to estimate the overall change in range of 
detection, it is necessary to determine the relative im- 
portance of the different methods. In general, visual 
detection ranks first, because of long range and ease 
of identification, radar second, and sonar third. Dur- 
ing a period near the end of World War II, for in- 
stance, United States submarines made about 800 
contacts by visual detection, 300 by radar, and 50 by 
sonar . German U-boats have not had as effective 
search radar as United States submarines, so that the 
importance of radar in U-boat operations has been 
much less. These figures would depend a great deal 
on submarine tactics: a submarine which spent most 
of the time submerged would make relatively more 
sonar contacts. It is thus evident that the contact 
range increases with an increased number of ships 
but is by no means proportional to it. A fairly rea- 
sonable approximation would be to consider the 
range as proportional to Then the expected 

number of contacts made will depend on two factors: 
this increased range and the decrease in the number 
of units at sea to be detected. 

Nc = N^x ni/3 X - 
n 

_ N, 

„2/3 (2) 

where 

Nc = expected number of contacts on convoys; 

Ni = expected number of contacts on same ships 
sailing independently. 


A change from n = 1 to n = 64 will reduce the num- 
ber of contacts made on a given number of ships at 
sea by a factor of 16, according to equation (2). 

This gain by convoying is somewhat enhanced by 
the greater ease of evasive routing of convoys. With 
a convoy system in operation, there are few units at 
sea and it is relatively easy to direct them so as to 
avoid known submarines or concentrations of sub- 
marines. This would not be so practical for inde- 
pendent vessels. Their number would be too great, 
their positions too poorly known, and their commu- 
nications inadequate. Hence available intelligence 
concerning submarine dispositions can be utilized 
most effectively with a convoy system. 

The decrease in contacts resulting from a convoy 
system usually causes the submarines to employ 
wolf-pack” tactics, in which any submarine making 
contact endeavors to inform others in the vicinity 
and home them to the convoy. For the second and 
subsequent submarines, the search is no longer at 
random, and the number of contacts made is in- 
creased by their additional knowledge. If half a 
dozen submarines can be homed to each original 
contact, the number of contacts made by each is ap- 
proximately six times as great as without homing. 
United States submarines operating in groups of 
about three have made about 1.7 times as many con- 
tacts as those hunting independently. 

German U-boats have formed wolf packs as large 
as 10 to 20 for some attacks, though their average size 
was, no doubt, much smaller. The general procedure 
was for the first U-boat making contact to report the 
convoy to U-boat control (and to other U-boats) and 
then to trail the convoy without attacking, supply- 
ing more information concerning the convoy as avail- 
able, and, when possible, supplying homing signals 
to aid other U-boats in closing the convoy. On re- 
ceipt of the contact report, control would direct other 
U-boats in the vicinity to intercept the convoy and 
attack it. Thus a group of U-boats would collect in 
the immediate vicinity of the convoy, each acting 
more or less independently. When a U-boat was in 
favorable position, an attack would be launched, 
and, after a brief retirement, the U-boat would en- 
deavor to get into position again for reattack. This 
procedure would often be kept up for several days 
and nights. In this way only a fraction of the total 
number of U-boats in the wolf pack would be in 
contact with the convoy at any one time, and only a 
fraction of them would be actively attacking. Never- 




102 


CONVOYING AND ESCORT OF SHIPPING 


theless, the ability to form a large attacking group 
when a convoy is sighted is of great importance in 
convoy battles, and much antisubmarine effort is de- 
voted to preventing tracking and wolf-pack for- 
mation. 

10.1.2 Submarine Approach to Convoy 

When the submarine has made contact with a 
convoy, an approach to within torpedo-firing posi- 
tion must be made. There are two aspects to the sub- 
marine’s problem: the natural difficulty of making 
such an approach, particularly serious when sub- 
merged, and the additional difficulty of avoiding de- 
tection and counterattack by the convoy’s escorts. 
The typical convoy being large, slow, and unmaneu- 
verable, the latter aspect is usually the more impor- 
tant. Only in the case of a fast convoy which the 
submarine cannot track or overtake on the surface 
is there any great difficulty in the approach process 
when the submarine is unopposed by escorts. 

As was seen to be the case for independent ships, 
speed has an important effect on the safety of con- 
voys. A speed sufficient to prevent tracking and over- 
taking makes it impossible for the submarine to get 
into position ahead by an end run, reduces the track- 
ing data which the submarine can obtain, and limits 
it to one quick approach and attack. In addition, 
high convoy speed makes it difficult to gather a wolf 
pack for the attack. The effect of speed cannot readily 
be broken down for analytical study, and presenta- 
tion of operational data on the overall effect of speed 
is deferred until a later section. The detailed evalua- 
tion of the effectiveness of escorts, both surface and 
aircraft, in detecting and preventing the approach of 
a submarine to a convoy, is discussed in Volume 2B, 
Search and Screening, since the screening problem is 
simply that of searching for a target known to be try- 
ing to approach a certain region. In Volume 2B, 
Chapters 8 and 9, methods are described for deciding 
on the optimum screening disposition or plan. Conse- 
quently, the problem of making such decisions will 
not be discussed here. The general method of anal- 
ysis, however, is as follows. 

The convoy is surrounded by a torpedo danger 
zone which includes all points from which the sub- 
marine has a fair chance of hitting one or more ships 
in the convoy (a chance of 25 per cent or greater, for 
example). Ahead of this zone is another called the 
submerged approach zone, bounded by the limiting 


approach lines on the sides and the submarine’s de- 
tection radius of the convoy on the front. Any sub- 
marine in this zone is considered to be aware of the 
presence of the convoy and can approach it sub- 
merged. To the flanks and rear is the surfaced ap- 
proach and tracking zone, in which submarines are 
in contact with the convoy but must remain on the 
surface to have sufficient speed to approach or track 
it. (For a fast convoy this zone also has a limiting 
after-bearing behind which the submarine cannot 
approach, even on the surface.) These zones are 
shown in Figure 2. 

The primary aims of aircraft escort are two. 

1.. To force down submarines in the surfaced ap- 
proach and tracking zone and cause them to lose con- 
tact with the convoy; and 

2. To detect submarines which would enter the 
submerged approach zone and prevent them from 
doing so. The submarines are to be forced down and 
immobilized so that the convoy may be turned away 
from the contact and thus avoid the submarine. 

A good aircraft escort plan should reduce by at 
least 50 per cent the number of submarines gaining 
access to the submerged approach or torpedo danger 
zones. In addition, tracking and wolf-pack formation 
are made very much more difficult. 

The chief aim of the surface craft screen is to de- 
tect submarines in the submerged approach zone and 
take them under counterattack before they can fire 


/ 

/ 


I 


\ 

\ 

\ 


SUB'S DETECTION CIRCLE 



SURFACED APPROACH AND TRACKING ZONE 


/ 

/ 


\ 


/ 


\ 








Figure 2. Submarine approach zones around a convoy. 


THE GAIN IN SAFETY BY CONVOYING 


103 


torpedoes. In addition, surface craft may take offen- 
sive action against submarines first contacted by air- 
craft and must be prepared to intercept surfaced 
approach and prevent tracking when air cover is not 
available. Their effectiveness in the latter role is 
difficult to estimate. With a normal size convoy, sonar 
screens may reasonably be expected to intercept 50 to 
75 per cent of the approaching submarines, assuming 
good sonar conditions and a normal number of 
screening ships. 

To compare the safety of a convoy with that of an 
independent ship on a theoretical basis, all these 
factors should be taken into account quantitatively. 
7 his can be done only in the most approximate sort 
of way. The aircraft screen should intercept at least 
50 per cent of the submarines approaching, and the 
surface craft at least 50 per cent of those not de- 
tected by aircraft. Other things being equal, the sub- 
marine would be able to carry through less than one- 
quarter as many approaches to escorted convoys as to 
independent ships per hundred initial contacts. The 
danger of attacking an escorted convoy is great 
enough to have an important psychological effect, as 
well. A strong escort may deter the submarine com- 
mander from even trying to attack. Actually, the 
large size and low maneuverability of a convoy cer- 
tainly simplify the submarine’s approach problem 
somewhat, and it seems probable that the ratio be- 
tween the fraction of successful approaches on con- 
voys and the fraction on independents would be 
nearer one-half than one-quarter. 

10.1.3 Torpedo Hits Slightly Easier 
to Achieve on Convoys 

Having achieved a firing position, the submarine 
still has the problem of securing one or more torpedo 
hits. It is evident that this will be easier with a con- 
voy than with an independent ship, since a large 
number of targets is presented. In general, the sub- 
marine will fire at a particular ship, but may fail to 
hit it and hit some other ship instead. A detailed dis- 
cussion of the calculation of the probability of such 
hits is discussed in connection with the mathematical 
formulation of the screening problem in Volume 2B, 
Chapter 8. A typical example will suffice to illustrate 
the principles involved. Consider the case of a sub- 
marine firing into a large convoy from abeam as 
shown in Figure 3. 

A torpedo fired from 2000 yd abeam can pass 


1000 YDS 



through several columns. If it is fired at random, its 
probability of encountering a ship while passing 
through the first column would be //800 where I is 
the length of a ship in yards. For / == 140 yd, the 
chance of a hit in passing through three columns 
(5000-yd range torpedo) would be 

Probability of hit = 1 — ^1 — 

= 44%. (3) 

If a total of four torpedoes is fired, the expected num- 
ber of hits is 4 X 0.44 = 1.76. Since not all hits will 
cause ships to sink, it is reasonable to estimate that 
1 to 1 1/^ ships would be sunk as a result of the attack.^ 
This is to be compared with a maximum of one ship 
in an attack on an independent ship; an average re^ 
suit would probably lie in the range from 1/2 to 1 ship 
sunk per attack on an independent. On this basis we 
would expect the ratio between sinkings per salvo 
fired at a convoy and sinkings per salvo fired at an 
independent to be not greater than three to one, and, 
more likely, about two to one. 

It is evident from Figure 3 that the number of ships 
sunk per salvo will not depend at all critically on the 
size of convoy. If the submarine uses a torpedo of 
5000-yd range, it can penetrate only about three or 
four columns, so that increase in convoy size to more 
than four columns causes no further increase in ships 
sunk per salvo. Only with very long-range torpedoes 
does this quantity continue to increase after a convoy 
size of 10 to 20 ships has been reached.^ 

a This figure depends on a number of factors, such as type of 
ship, distance from land, roughness of sea, etc. The experience 
of United States submarines has been that about 40 per cent of 
the ships hit by one torpedo sink. This would lead to 1.76 X 0.40 
= 0.71 ships sunk per salvo. Thus the estimate of 1 to U/^ ships 
sunk is probably too high, if anything, though it may be about 
correct for rough weather in mid-Atlantic. 

t> As a corollary it may be concluded that for attack on very 
large convoys, a long-range torpedo would be very effective be- 
cause it would have good chances of success when fired into the 
convoy at random as a brawning shot. 




104 


CONVOYING AND ESCORT OF SHIPPING 


Table 1. Convoy losses in North Atlantic, August 1942 -January 1943. 



Convoys 

Ships 

Convoys 
sighted 
by U-boat 

Ships sunk 

Per cent 
convoys 
sighted 

Per cent 
ships 
sunk 

HX (eastbound 9i/^ kt) 

23 

923 

8 

12 

35 

1.3 

SC (eastbound 7 kt) 

24 

991 

14 

45 

58 

4.6 

ON (westbound 91/2 kt) 

24 

897 

11 

29 

46 

3.2 

ONS (westl)ound 7 kt) 

23 

836 

11 

31 

48 

3.7 


10.1.4 Overall Value of Convoying 

From these figures a very rough estimate of the 
overall gain of safety by convoying can be made. A 
typical average size of convoy during the Battle of the 
Atlantic was about 30 ships. Transatlantic convoys 
were often larger, coastal convoys normally smaller. 
For the comparison three factors must be taken into 
account. 

1. The ratio of contacts made on convoys to those 
on independent ships for the same number of ships 
at sea, which according to equation (2) is given by 
( 30 ) -2/3^ or about one-tenth. 

2. The relative difficulty of making the approach 
to firing position. It was estimated that the submarine 
would be successful about half as often with convoys. 

3. The fact that sinkings per chance to fire a salvo 
are probably about twice as great with large convoys. 

The net effect of these factors is: 

Relative loss in convoy “ Jq "2 ^ ^ 

( 4 ) 

This is a considerable gain in safety and is in rea- 
sonable agreement with the observed difference in 
loss rate given in Part I. The loss per month in con- 
voy was 4 per cent during early 1942 in the United 
States Strategic Area, while that for independent 
ships was 20 per cent, a ratio of one to five. This ratio 
is somewhat less than that estimated in equation (4), 
which is surprising because equation (4) should give 
a fairly conservative estimate of the gain involved in 
convoying. One possible explanation of the discrep- 
ancy is the emphasis placed by the Germans on wolf 
packs, by which the advantages of convoying can be 
largely neutralized. If a pack of ten submarines were 
collected on each contact and all were effective, the 
loss in convoy, based on equation (4) would be the 
same as for independent ships, rather than one-tenth 
as great. Thus the operational ratio of one-fifth 


might be interpreted as implying that the Germans 
were able to home an extra U-boat to each contact, 
on the average. In many cases, wolf packs of more 
than two U-boats were formed, but in many other 
cases only one U-boat was in contact. The average 
number of U-boats attacking each contact would be 
two, which can be compared with the number 1.7 
which United States submarines have achieved while 
operating in groups (of about three).<^ 

It may be concluded that there is no irreconcilable 
difference between our theoretical estimate of the 
situation and operational data for the period quoted, 
which data are typical of operational results. This 
overall agreement confirms the belief that convoying 
is probably the most effective method of reducing 
sinkings but does not prove that the foregoing pic- 
ture of the mechanism by which sinkings are reduced 
is correct. In order to throw further light on the 
details of the process, it is necessary to analyze the 
operational data more carefully and determine the 
effect of factors such as size and speed of convoy and 
strength of escort, on the losses of ships from convoy. 
A number of such studies are discussed in the follow- 
ing section and are correlated with the rough theory 
given above. 

102 OPERATIONAL STUDIES OF THE 
EFFECT OF VARIOUS FACTORS 
ON THE SAFETY OF SHIPS 
IN CONVOY 

10 . 2.1 The Effect of Convoy Speed 

Speed has a very considerable effect on the safety 
of independent ships and might therefore be pre- 
sumed to have a similar effect for convoys. There 

c A figure analogous to the five to one ratio found for inde- 
pendent versus convoyed ships can be obtained by comparing 
sinkings before and after convoying in certain regions. Ihe 
Capetown and Trinidad areas (both regions of high U-boat ac- 
tivity) give ratios of about six to one and ten to one. 



EFFECT OF VARIOUS FACTORS ON SAFETY OF CONVOYS 


105 


Iiave been, however, relatively few convoys of speeds 
from 10 to 20 knots, in which speed range the out- 
standing effects are observed with independents. 

In order to have for study a large number of con- 
voys with serious exposure to submarines, it is neces- 
sary to deal with North Atlantic convoys, whose 
maximum speed was about 9 1/9 knots. Data on these 
convoys for the G-month period from August 1942 
through January 1943 are presented in Table 1. Dur- 
ing this period 80 to 110 U-boats were operating in 
the North Atlantic and convoy losses were high. 

From Table 1 it can be concluded that (1) the effect 
of speed in the success of the attacks is quite clear in 
eastbound and unclear in westbound convoys, and 
(2) that an effect of speed on sightings may be present 
but is not strongly indicated. The discrepancy be- 
tween eastbound and westbound convoys is very 
striking and makes interpretation of the data 
difficult. 

In an attempt to explain this difference, the routes 
of a fair sample of convoys (omitting those for ON 
convoys which traveled very much to the south be- 
cause of containing a portion bound for Africa) were 
reconstructed. The mean eastbound and westbound 
tracks are given in Figure 4. The westbound convoys 
evidently have traversed a route some 200 to 400 
miles south of the mean eastbound route between 
20°\V and 50° W. 

A plausible explanation of the different effects of 
speed on eastbound and westbound convoys follows: 
Eastbound convoys being farther north received bet- 
ter air cover since aircraft flew from Iceland. Conse- 
quently U-boats operating against them had to spend 
a larger fraction of their time submerged. The meaji 
speed of U-boats is therefore less when attacking 



Figure 4 . Average convoy routes in the North Atlantic, 
August 1942 through January 1943. 


eastbound than westbound convoys. Consequently 
the increase of speed from 7 to 91/9 knots may be a 
critical range and give large loss reduction for east- 
bound convoys, though not for westbound. This line 
of reasoning is confirmed by the fact that aircraft 
made 40 attacks on U-boats in connection with east- 
bound convoys and only 5 with westbound. 

What is really needed to make the effect of speed 
quite clear is a set of transatlantic convoys all of 
which have received approximately the same air 
cover. Those convoys which travel a southerly route 
in general are not covered, and, as was shown in 
Figure 4, this has applied more to westbound than 
to eastbound convoys. If we eliminate all convoys 
which make the mid-ocean part of the voyage south 
of the Great Circle route, the remaining ones should 
be fairly homogeneous with respect to air cover and 
should give an indication of the value of speed under 
those conditions. For this purpose the routes of all 
transatlantic convoys from October 1942 to early 
May 1943 were plotted out and those which spent 
more than half their time (between 15° W and 
45 W) south of the Great Circle route eliminated. 
The remainder is shown in Table 2. Stragglers are 
included in the figures. 


' Table 2. Losses for convoys on northerly routes. 



Number 

of 

Per cent 

Per cent 

Ships 
sunk per 
convoy 


convoys 

sighted 

attacked 

sailing 

SC and ONS (7 kt) 

36 

68 

43 

1.9 

HX and ONF (9i/^ kt) 

44 

68 

41 

1.3 


d For comparison, the data on convoys which spent more than 
half their time south of the Great Circle are shown. 


Losses for convoys on southerly routes. 



Number 

of 

Per cent 

Per cent 

Ships 
sunk per 
convoy 


convoys 

sighted 

attacked 

sailing 

SC and ONS (7 kt) 

8 

88 

88 

2.9 

HX and ONF (9i/^ kt) 

7 

86 ' 

86 

3.7 


The numbers here are small and actually the fast convoys 
suffered more heavily than the slow. It is of great interest that 
they were attacked twice as often as those on northerly routes, 
particularly since the number of U-boats on patrol in the south- 
erly zone was less than a third of the number further north. 


106 


CONVOYING AND ESCORT OF SHIPPING 


Table 3. Losses for various sizes of couvoys, 1941-1942 
(North Atlantic Allied convoys — wolf-pack attacks). 


Size of 
convoy 

Number of 
pack attacks 

Average 
number of 
ships 

Average 
number of 

escorts 

Average 
size of 

U-boat pack 

Average 
number of 
ships sunk 
per attack 

Ships sunk 
per U-boat 
attacking 

0-14 

1 

11 

4 

4 

7 

1.8 

15-24 

8 

20.4 

6.5 

6.5 

4.8 

0.7 

25-34 

11 

29.7 

6.8 

5.1 

5.6 

1.1 

35-44 

13 

38.5 

6.1 

5.8 

6.1 

1.1 

45-54 

7 

48.3 

6.5 

5.2 

4.9 

0.9 

55 and over 

2 

62.5 

8.0 

7.5 

9.0 

1.2 


The number of convoys is large enough that the 
data may be considered significant. There is no differ- 
ence in the frequency of sightings or attacks, but the 
number of ships sunk per fast convoy is two-thirds of 
that per slow convoy. 

The overall conclusion is that some curve similar 
to that of Figure 3 in Chapter 9 obtains for convoys, 
but it is probably displaced in the direction of lower 
speeds. Air cover tends to cause the critical region of 
the curve to occur at slower speeds, and the shape of 
the curve depends on the extent of aircraft escort. 
The data available are very far from adequate to es- 
tablish the detailed nature of this curve and its de- 
pendence on the air escort provided. 

10.2.2 'pjjg Effect of Convoy Size 

It might very reasonably be claimed that the effect 
of convoy size is the most important phenomenon 
associated with convoying; it is, in fact, the essence of 
convoying, since an independent vessel can be consid- 
ered to be a convoy of one ship. On the other hand, 
the presence of escorts might be considered the most 
important feature of the convoy system, and this 
feature will be discussed immediately after considera- 
tion of the effect of convoy size. 

Several studies have been made of the losses suf- 
fered by North Atlantic convoys as a function of size 
and, although the losses were fortunately too small to 
provide data of unquestionable statistical signifi- 
cance, they lead to some very interesting conclusions. 
In the first place, data are presented in Table 3 per- 
taining to convoys which were attacked during 1941- 
1942 and in which the approximate number of at- 
tacking U-boats was known. 

The striking feature of Table 3 is the constancy of 
the figures in the last column. There is certainly no 


clear tendency for the number of ships sunk to in- 
crease with size of convoy. It might be expected that 
there would be a natural tendency in that direction 
which would be counterbalanced by an increased 
strength of escort with the large convoys, but the 
figures given do not bear out such a contention. For 
convoys of from 15 to 55 ships the strength of escort 
was virtually constant, but no appreciable change of 
sinkings with size of convoy is observed. Hence the 
earlier theoretical conclusion that increase in convoy 
size to more than four columns causes no further in- 
crease in ships sunk per salvo is borne out by the 
operational data. A convoy of 15 or more ships is 
apparently large enough that the number of ships 
sunk per U-boat attack does not depend on convoy 
size. Such would not be the case for smaller convoys, 
where we would expect increased size to be associated 
with an increase in number (though not in fraction) 
of ships sunk. Data on small convoys are available 
from attacks by United States submarines on Jap- 
anese convoys. For the period from July 1, 1942, to 
to March 1, 1943, the results are given in Table 4. 


Table 4. Sinkings from Japanese convoys as a function of size. 
(By U. S. submarines.) 


Size of convoy 

Total number 
of convoys 

No. of M/V 
sunk 

M/V sunk 
per convoy 

1 

1222 

276 

0.23 

2 

400 

142 

0.36 

3 

243 

103 

0.42 

4 

174 

79 

0.45 

5 

98 

70 

0.71 

6 

74 

47 

0.64 

7 

33 

28 

0.87 

8 

27 

16 

0.60 

9 

14 

13 

0.93 

10 

15 

16 

1.07 

11-12 

23 

16 

0.70 

13-14 

12 

19 

1.6 

15-20 

7 

8 

1.1 


EFFECT OF VARIOUS FACTORS ON SAFETY OF CONVOYS 


107 


1.5 


1.0 


0.5 


X 

X 

X 

O 

O 


O 

x/ 

X / 

X 

/x 

O 

O 

GERMAN U/B / 
ALLIED CO 

ATTACKING 

iNVOYS 

7 U.S. SUBS 
/ JAP C 

ATTACKING 

JONVOYS 




NUMBER OF SHIPS IN CONVOY 


Figure 5. Effect of convoy size on ships sunk per sub- 
marine attack. 


1942, to March 31, 1943. A total of 160 ships was 
sunk. The pertinent data are summarized in Table 5. 


Table 5. Frequency of attacks and sinkings as a function 
of convoy size. 

(1942-1943, Western North Atlantic.) 




Average 




Average 

number of 

Average 

Average 

Number of 

number of 

sinkingsf 

number of 

number of 

merchant 

attack-days* 

per 

sinkings 

escorts 

vessels 

per convoy 

attack-day 

per convoy 

per convoy 

10-29 

0.5 

1.9 

0.9 

6.5 

30-49 

0.4 

2.7 

1.0 

6.4 

50-69 

0.5 

2.3 

1.0 

6.8 


An attack-day is defined as a day on which a sinking occurred, 
t Sinkings in convoy only; stragglers are excluded. 


With smaller convoys, there is a definite increase 
in the number of merchant vessels sunk as the size 
of convoy increases, which is in accordance with ex- 
pectations. A comparison of the two sets of data is 
presented in Figure 5. The two are not directly com- 
parable, since the sizes of convoys involved do not 
overlap and since the effectiveness of escort in the 
two cases is undoubtedly different. Nevertheless, the 
overall picture of a considerable increase for small 
convoys followed by an inappreciable increase for 
sizes above 20 ships is quite clear. 

So far attention has been devoted to the ships sunk 
per submarine attack, omitting the frequency with 
which the attacks occurred. From the data presented 
it appears that large convoys are much the safest for 
the individual ships, since the fraction of convoyed 
ships sunk decreases markedly with increasing con- 
voy size. If, however, it were much easier for a sub- 
marine to make an attack on a large convoy than on 
a small one, by virtue of the greater ease of contact 
and approach, there would not necessarily be any 
overall gain in the adoption of large convoys. In 
order to determine whether the preceding results 
give a true picture of the net effect of changes in con- 
voy size, the problem must be studied in more general 
terms. 

This was done by selecting a fixed region (the 
United States Strategic Area east of 50° W) in which 
the U-boat activity was high and by determining the 
losses suffered by transatlantic convoys in crossing 
this area as a function of convoy size. A total of 114 
convoys were studied; they spent an average of about 
7 days in the area. The period covered is from July 1, 


Again the losses suffered by the convoys do not de- 
pend on size of convoy. On the average, approxi- 
mately one ship was sunk from each convoy crossing 
the region, regardless of its size. Since the convoys in 
the 50 to 69 group were about three times as large as 
those in the 10 to 29 group, each ship in the large con- 
voys was exposed to only about one-third the danger 
to which each was exposed in the smaller ones. 

10.2.3 Yhe Value of Surface Escorts 

An outstanding feature of the convoy system is the 
provision of protective forces, both surface craft and 
aircraft. Surface craft will be discussed first. These 
antisubmarine ships have a number of functions: (1) 
detection of submerged submarines at the front of 
the convoy and of surfaced submarines approaching 
from any bearing, (2) offensive sweeps to harass, 
locate, and put down submarines in the vicinity 
which are trying to trail the convoy or get into posi- 
tion ahead of it, and (3) attacks on submarines when 
opportunity presents. Escort of convoy is more than 
a matter of defensive screening, and the most suc- 
cessful escort group has been the one which prevents 
the enemy from ever obtaining the initiative and 
launching a determined attack on the convoy. The 
conflict between the group of escorts and an attack- 
ing wolf pack is one which cannot be simply de- 
scribed but involves the skillful use of radar, sonar, 
visual detection, radio direction finding, available 
intelligence, and weather conditions, and an avoid- 
ance of routine and stereotyped tactics. An endeavor 
to estimate the value of escorts on a theoretical basis 


108 


CONVOYING AND ESCORT OF SHIPPING 


would be a hopeless task, but an interesting indica- 
tion of their importance can be obtained from opera- 
tional data. 

Numbers corresponding to those of Table 3 are 
given in Table 6 for 1941-1942 in the North Atlantic. 


Table 6. Merchant ship losses as a function of escort 
strength. (1941-1912 in North Atlantic). 


Escort 

strength 

Number of 
pack attacks 

Mean escort 
strength 

Ships sunk per 
U-boat in pack 

1-4 

22 

3.4 

0.88 

5-9 

51 

6.7 

0.75 

10-15 

75 

11.1 

0.34 


The final column gives the average number of ships 
sunk in an attack per U-boat present in the attacking 
wolf pack. This figure is a measure of U-boat success 
in penetrating the screen and sinking ships. The 
chief effect of surface escorts might be expected to be 
in preventing the U-boat from reaching firing posi- 
tion. In very simple terms we can interpret the figures 
(plotted in Figure 6) in the following way. 

1. Assume that in the absence of any escorts the 
U-boat can make a virtually unopposed approach 
and sink 1.2 ships. (Earlier it was estimated that a 
salvo would sink 1 to 1.5 ships.) When escorts are 
present, their effect is considered to be one of reduc- 
ing the fraction of cases in which the submarine 
reaches firing position undetected, but the number of 
ships sunk if it does so is not changed. 

2. The effect of each additional escort is to reduce 
the ships sunk by about 0.075 ship, that is, to reduce 
the U-boat's chance of penetrating the screen by 
about 6 per cent. Apparently, 16 escorts would give 



Figure 6. Effect of number of escorts on convoy losses. 



PERIMETER TO DEFEND ABOUT 
30 MILES 

Figure 7. Zone to be defended by escorts. 


complete protection. During the period under study 
most attacks were made at night and the sort of per- 
imeter which the escorts had to defend is shown in 
Figure 7. If 16 escorts are distributed around this 
perimeter, it can be seen that each one effectively 
screened about 2 miles of it or, in other words, had 
an effective detection range of about 1 mile on the 
U-boats under those conditions (surfaced night at- 
tacks predominating; most ships fitted with radar, 
but not all centimeter type; North Atlantic wolf 
pack operations). 

It is rather surprising that there is no evidence of 
an upward concavity in the curve of Figure 6. It does 
not seem likely that even 16 or more escorts could 
completely prevent submarines from sinking any 
ships, but it does seem likely that the number of 
ships sunk would be made very small, perhaps 0.1 or 
0.2, for large numbers of escorts. On the other hand, 
it may be that the presence of a large number of es- 
corts is sufficient to discourage U-boats from aggres- 
sively pressing home attacks. Perhaps they would 
consider it too dangerous to attack a convoy with 15 
or more escorts ever to try to do so, even though they 
might actually have some chance of success. The data 
available are sufficient only to suggest this question, 
not to answer it. They do, however, show conclu- 
sively that strength of escort played a very important 
role in determining ship losses.® 

The figures of Table 6 are given on a per attack 

e This contrasts strongly with the experience of United States 
submarines in attacking Japanese convoys, for their ability to 
sink ships has not been affected by the number of convoy escorts 
present; the only possible conclusion is that Japanese detection 
equipment and procedure have been highly ineffective. 


THE IMPORTANCE OF LARGE CONVOYS 


basis, however, and do not take into account the 
frequency of attacks. Since surface escorts are often 
used for offensive sweeps designed to shake off trail- 
ing U-boats and prevent attack, such figures may not 
be a complete measure of the escort’s value. Unfor- 
tunately, it is not possible to determine from opera- 
tional data the ability of the escorts to prevent attack. 
One complicating feature is the tendency to provide 
more escorts when and where the danger is greatest, 
which increases the relative number of attacks made 
on convoys when many escorts were present. Actually 
the data of Figure 6 may be considered a conservative 
indication of the value of surface escorts. 

10.2.4 Yhe Value of Aircraft Escorts 

Similar information on the value of aircraft can be 
obtained by comparing the losses suffered by aircraft- 
escorted convoys with those sustained by convoys 
without such escort. Data are presented in Table 7 
for North Atlantic convoys from August to December 
1942. Only the days (and nights following) on which 
U-boats were known to be in contact are included, so 
that all figures pertain to threatened convoys. 


Table 7. Ship losses as a function of air escort. 
(August to December 1942, North Atlantic). 





Average 

■ 



Average 

size of 

Ships 


Ships 

number 

U-boat 

torpe- 

Number 

torpe- 

of 

pack in 

doed 

of days 

doed 

sorties 

contact 

per day 

With air cover 38 

23 

4 

4.9 

0.60 

Without air cover 43 

75 

0 

5.5 

1.75 


Since the days with and without air escort pertain 
to the same convoys and are in the same period, with 
approximately the same size of wolf pack in contact, 
we can feel quite confident that the difference be- 
tween 0.60 and 1.75 ships torpedoed per day is actu- 
ally due to the aircraft escort. The number of ships 
torpedoed is reduced to about 30 per cent of the 
value that it would otherwise have by the presence of 
such escort (an average of four sorties per day staying 
about 2 hours with the convoy). This figure, of 
course, applies to convoys encountering wolf packs 
of surfaced U-boats and must be used with caution. 
In addition, there is an interrelation between convoy 
speed and the effect of aircraft escort which was pre- 
viously noticed in connection with analysis of speed 


109 



Figure 8. Effect of aircraft flying and convoy speed. 

effects. Figure 8 is an effort to present the probable 
form of the relationships. 

The curves are drawn for various amounts of air- 
craft escort flying. The general belief is that the pri- 
mary effect of aircraft flying is to reduce the speed of 
convoy for which the submarine can carry out 
Method B tracking and approach, because it is forced 
to submerge at least part of the time. If we assume 
that 24 hours per day aircraft flying will eliminate 
tracking even for the slowest of convoys, then the 
curves drawn may be taken as representing 0, 6, 12, 
18, and 24 hr of aircraft flying per day. The plotted 
X corresponds to the data of Table 7 (average speed 
of convoy about one-half surfaced U-boat speed, loss 
rate reduced to 30 per cent). Since the point is for 8 
hr per day of flying and lies suitably enough between 
the 6-hr and 12-hr curves, the operational data can be 
considered in agreement with the curves which were 
sketched in on the basis of very qualitative argu- 
ments. It may be concluded that the curves of Figure 
8 give a good indication of the effect of aircraft escort 
and convoy speed on the losses of ships from convoys, 
but data available are not sufficient to demonstrate 
their exact nature, and they can be based only on 
indirect and devious reasoning. 

10.3 XHE IMPORTANCE OF LARGE 
CONVOYS 

It is evident from the preceding discussion that 
ships are safest when sailing in large, well-protected 
convoys. Although this fact may seem trivially obvi- 
ous, a quantitative estimation of the overall import- 
ance of large convoy size is of interest. 


110 


CONVOYING AND ESCORT OF SHIPPING 


For purposes of comparison, consider a situation 
in which a total of ten ships and one escort are ready 
to sail each day and calculate the losses for three 
cases: 30-ship convoys, 60-ship convoys, and 90-ship 
convoys. A convoy spends 6 days in the dangerous 
region, and a total of 12 hours per day of flying can be 
done actually escorting convoys. Convoy speeds are 
assumed to be one-half the submarine’s surfaced 
speed. The tactical situation is presented in Table 8. 


Table 8. Comparison of convoy sizes. 



I 

Case 

II 

III 

No. of ships in convoy 

30 

60 

90 

No. of escorts with convoy 

3 

6 

9 

Days between convoys 

3 

6 

9 

Convoys in danger area 

2 

I 

2 

Flying hours per convoy day 

6 

12 

18 


Correspondingly, a table can be made of the con- 
tributions to relative loss rate of each of the factors 
involved: convoy size, number of escorts, and extent 
of aircraft flying. This is done in Table 9. 


Table 9. Relative loss rates. 



I 

Case 

II 

III 

Effect of convoy size [Fig. 5 and eq. (2)] 

1.00 

0.69 

0.56 

Effect of number of escorts (Fig. 6) 

1.00 

0.75 

0.55 

Effect of aircraft flying (Fig. 8) 

1.00 

0.50 

0.30 

Overall 

1.00 

0.26 

0.09 


Thus it may be concluded that ships in large con- 
voys are very much safer than those in small or 
medium-sized ones under the conditions of this ex- 
ample. A ship’s chance of being sunk in a 90-ship 
convoy is about one-tenth that in a 30-ship convoy. In 
order to achieve full effectiveness from convoying as 
a means of protecting ships, it is of utmost import- 
ance to make the convoys large and well-defended. 
There are certain practical limitations to the size of 
convoy that can be sailed, but these results show that 
for maximum safety of ships convoys should be made 
as large as possible, larger, in fact, than they normally 
have been, even in the Atlantic. 

The chief difficulties involved in very large con- 
voys are: 


1. Increased difficulty of communications within 
the convoy. Signals from the commodore’s ship must 
be passed from one ship to another along rows and 
columns, and the passing of visual signals by mer- 
chant ships in a large convoy leaves much to be de- 
sired. Ships in convoy maintain radio silence, and a 
secure system of intraconvoy signaling has never 
been available. It is possible that the maintenance of 
radio silence may not be justified when the losses 
through faulty communication are balanced against 
the gains due to evasion of U-boats. 

2. Decreased maneuverability of convoy. With 
large convoys turns are difficult in any case and the 
ease of maneuvering is not a critical function of size. 

3. Increased number of stragglers. It may be 
argued that a greater fraction of ships will straggle 
from large convoys, primarily because of (1) and (2) 
above, and, since danger to stragglers is high, this 
may seriously increase the losses suffered by large 
convoys. Operational data, however, indicate that 
this effect is not large, probably because the chief 
reasons for straggling are engine breakdown or simi- 
lar failures which are in no way related to convoy 
size. 

4. Increased port congestion. To some extent any 
convoy system crowds harbor facilities, but the diffi- 
culty is increased the larger the convoys are. To the 
extent that ships spend unnecessary days in port, they 
are not available for carrying cargo and their value is 
reduced. The turn-around time in port is made up 
of: 

a. Waiting for berths to load and discharge. 
(This applies mainly to ships handling dry 
cargo, not to tankers.) 

b. Discharging and loading. 

c. Waiting in the channel for a convoy to sail. 
Period (a) will usually be kept fairly small after the 
convoy system has operated for a time but may be 
extended if the size of the convoy is increased above 
what has been a working average. Period (b) depends 
on the nature of the cargo and varies between 2 and 
20 days, as a rule. Period (c) averages half the time 
between convoys (“convoy cycle’’). Hence, an increase 
in convoy size will somewhat increase the turn-around 
time. 

The ultimate limitation on convoy size is thus the 
effect of increased size on the time spent in port and 
consequent slowing down of the actual transport of 
goods. If, for example, the average ship spends 10 


LIMITATIONS ON CONVOYING 


111 


days at sea and 15 days in port for an average convoy 
size of 30 ships, but 20 days in port for a convoy size 
of 90 ships, the increase in convoy size reduces the 
rate of cargo transport by 20 per cent. This is equiv- 
alent, as far as cargo carrying capacity goes, to an 
immediate sinking of 20 per cent of the ships in- 
volved and cannot be tolerated except in cases where 
the expected number of ships saved by the increase 
in convoy size is comparably great. 

104 LIMITATIONS ON CONVOYING 

It has been shown that a convoy system greatly in- 
creases the safety of ships at sea, especially if convoys 
are large and well protected. The gain is accom- 
panied, however, by a loss in cargo carrying capacity 
of the ships available, and convoying is by no means 
universally desirable. There are two main ways in 
which convoying slows down cargo transport: the 
increased time spent in port and a decreased speed 
of ship which results in increased time spent at sea. 

An analysis of United States coastal trade convoys 
was made, using the ships at sea during June 1943 as 
a sample. It was found that on the average a con- 
voyed ship spends 43 per cent of its time in port and 
57 per cent at sea. Of the time in port, 46 per cent is 
spent in waiting for convoys to form. Of the time at 
sea, 19 per cent could be saved by allowing the vessels 
to proceed independently at their rated loaded 
speeds. Consequently, if all these ships were routed 
independently, the same amount of goods could be 
transported within the shipping system in 69 per 
cent of the time required with convoys (which were 
rather small and had a cycle of about 5 days). Hence, 
the cargo carried by the convoyed ships was only 69 
per cent of what could have been carried by them if 
they had been sailing independently. 

Suppose, however, that the ships are routed inde- 
pendently in order to speed transport. The number 
of ships sunk per month is increased, and fewer ships 
are available. By the time that 31 per cent of the 
ships have been sunk, the situation would no longer 
appear favorable. The cargo carried per month by 
convoyed ships and independent ships would be as 
shown in Figure 9. (Loss rates assumed are 4 per 
cent per month at sea for convoys, 20 per cent for 
independents; convoyed ships at sea 57 per cent of 
the time, independents, 67 per cent.) After about 3 
months, convoying begins to pay dividends in terms 
of greater cargo carrying capacity. 



TIME IN MONTHS FROM START OF M/V LOSSES 

Figure 9. Cargo carrying capacity of ships during sub- 
marine offensive. 

In deciding on the desirability of convoying, how- 
ever, the total cargo carried is the chief item of in- 
terest. After 3 months the independent ships would 
still have carried more cargo because they had a con- 
siderable initial advantage. It would not be until 
after about 6 or 7 months that the convoyed ships 
would have the larger total. This total is plotted in 
Figure 10. 

Thus the overall value of convoying in any par- 
ticular situation depends on how much longer the 
war is going to last, among all other things. For the 
conditions represented by Figure 10, independent 
ships would produce the best result as long as the 
war was not likely to last more than 7 months. 



TIME IN MONTHS FROM START OF M/V LOSSES 

Figure 10. Total cargo carried. 


“7:5 


112 


CONVOYING AND ESCORT OF SHIPPING 


whereas convoying would be desirable in a longer 
war. In addition, the bad effects of failing to convoy 
in the latter case appear somewhat more serious than 
the bad effects of convoying when one should not do 
so, which may be reason for advocating a generally 
conservative procedure. The overall conclusion is. 


then, that convoying is a powerful method for pro- 
tecting ships, but that it should not be applied unless 
the seriousness of the enemy’s submarine offensive 
and the probable duration of the war justify it. If 
the danger from submarines is fairly great, then large 
convoys should be formed. 


Chapter 1 1 

ATTACKS BY SURFACE CRAFT 


TTThen an antisubmarine ship or aircraft makes 

^ V contact with a submarine, an attack is nor- 
mally made in order to sink or damage it. During 
escort operations, “embarrassing,” or “urgent,” at- 
tacks may also be made whose chief purpose is to 
frighten the submarine crew and prevent it from 
pressing home its attack on the convoy. This type of 
attack will not be considered in the following discus- 
sion, since its value is largely psychological and can- 
not readily be evaluated in quantitative terms. The 
so-called deliberate attack, on the other hand, aims to 
destroy the submarine, and this aim can be expressed 
mathematically. In designing attack weapons and 
tactics the objective is to make the probability of 
destruction a maximum. The problems which arise 
in doing so will now be analyzed. 

1 GENERAL STATEMENT OF 
PROBLEM 

As is true in the other aspects of antisubmarine 
warfare [ASW] discussed in the previous chapters, 
the basic principles of attack are simple to describe. 
It IS only when these principles are examined in more 
detail that complicated problems arise. So before 
passing to the details of the attack consider the over- 
all picture of a surface craft attack against a subma- 
rine, first in terms of an a priori analysis and then in 
terms of an a posteriori analysis of operational re- 
sults. These two points of view will also be employed 
in the later detailed discussion. 

11.1.1 Theoretical Analysis 

From the theoretical point of view we are inter- 
ested primarily in attacks against submerged sub- 
marines, though an antisubmarine action often in- 
volves gunfire or ramming when the submarine is 
surfaced. The submarine may have been detected on 
the surface initially, or it may have been forced to 
the surface by previous attacks. In either case the 
ensuing action on the surface is little different from 
any other surface action and needs no consideration 
here. 

When the presence of a submerged submarine has 
been detected by sonar, the initial step in the attack 



is to localize the submarine, i.e., to determine its 
range and bearing from the attacking ship. On the 
basis of continuing range and bearing data, the sub- 
marine must then be “tracked” in order to determine 
its course and speed. Sometimes this is done explicitly 
by plotting positions, but more often it is done im- 
plicitly.*^ Finally, the attacking ship must maneuver 
into a position such that when it launches its explo- 
sives they will reach a point beneath the surface at 
the same time as the submarine reaches that point. 
Figure 1 illustrates a typical attack in which the bar- 
rage is laid off the stern and to the quarters of the 
attacking ship, and Figure 2 illustrates an attack in 



a Standard doctrine presented in FTP 223A for the use of 
cut-on technique and range recorder is an example of an im- 
plicit tracking procedure. Range rates, recorder traces, and 
changes in bearing are used to give rules for carrying out the 
attack. The antisubmarine attack plotter, on the other hand, is 
a device for explicit tracking, since it presents a geographic plot 
of submarine motion. 


113 


114 


ATTACKS BY SURFACE CRAFT 


which the explosives are thrown ahead of the attack- 
ing ship. These explosives may be activated by con- 
tact fuzes as in the case of Mousetrap or Hedgehog 
projectiles; by proximity fuzes, as in the case of Mk 8 
and Mk 14 depth charges; or by depth fuzes, as in the 
case of conventional depth charges or “Squid.” If the 
attacks illustrated in Figures 1 and 2 are to be success- 
ful, the charges must explode sufficiently close to the 
submarine either to rupture its pressure hull and 
cause immediate sinking or to damage the hull suffi- 
ciently to force it to the surface where it can be sunk 
by gunfire or ramming. 

Operational Studies 

Theoretical investigations usually involve a de- 
tailed analysis of the attack problem, but this is not 
possible in the case of studies based on operational 
data because of the nature of the information avail- 
able. The primary source of data for operational 
analysis is the action report prepared by each vessel 
which has made an attack. All attacks made on the 
same submarine during a more or less continuous 
engagement are grouped as one incident, whose re- 
sults are assessed by the appropriate committee. The 
assessments are graded A to J as follows. 

A. Known sunk. 

B. Probably sunk. 

C. Probably damaged, possibly sunk. 

D. Probably damaged. 

E. Probably slightly damaged. 

F. Insufficient evidence of damage. 

G. No damage. 

H. Presence of submarine uncertain. 

I. Target attacked not a submarine. 

J. Insufficient evidence to assess. 

Assessments of this type have a number of limita- 
tions as a measure of the success of the attack. In the 
first place, there is often some uncertainty as to 
whether the target attacked really was a submarine. 
If the submarine is not seen at any time, it is difficult 
to resolve this uncertainty on the basis of sonar data, 
and many attacks must consequently be assessed H. 
The usual solution is to eliminate from any analysis 
all attacks assessed H, I, or J. This is by no means a 
perfect solution, however, for a submarine is prob- 
ably present in a considerable number of H attacks. 
Assessment of damage is also somewhat uncertain 
since debris and other visible evidence may not in 


any particular case give a very accurate indication of 
the actual damage inflicted on the submarine. For 
purposes of analysis the assessments are usually 
grouped in the following categories. 

Sunk A, B 

Damaged C, D, E 

Undamaged F, G 

Nonsubmarine H, I, J 

As shown in Appendix I, the total number of A and 
B assessments did actually correspond closely to the 
total enemy submarine losses during World War II. 

The other data concerning the attack are often less 
reliable than the assessment. The submarine’s be- 
havior is completely unknown, as a rule, and data 
from the attacking ship are likely to be undepend- 
able. Ranges, bearings, and times are recorded in the 
heat of battle or later from memory, and in either 
case are likely to be inaccurate (if they are, in fact, 
recorded at all). For this reason the recorded data 
cannot be used as a basis for a reconstruction of the 
attack with any high hopes of accuracy. Even in prac- 
tice attacks, where the data are taken more carefully 
and submarine maneuvers are known, it is almost 
impossible to determine within a reasonable margin 
for error how close the barrage came to the subma- 
rine without some special device for doing so. The 
detailed course of events during an attack on any 
enemy submarine can rarely be determined, so that 
operational analysis consists of evaluating statisti- 
cally the effect of different changes in conditions or 
methods of attack on the overall success as embodied 
in the incident assessment. Results of such opera- 
tional studies can then be compared with those of 
theoretical investigations based on a detailed consid- 
eration of the factors influencing probability of suc- 
cess. In order to do this the importance of some of 
these factors must first be indicated. 

11.2 THEORETICAL DISCUSSION OF 
FACTORS DETERMINING THE 
SUCCESS OF ATTACKS 

The factors determining the probability of success 
in an attack can be grouped in two general cate- 
gories: attack errors on the one hand and weapon 
lethality on the other. The first problem encountered 
in examining the attack errors in detail is the esti- 
mation of errors involved in localization. Ranges and 
bearings on the submarine are obtained by sonar. 


FACTORS DETERMINING THE SUCCESS OF ATTACKS 


15 


which is subject to certain disturbances and limita- 
tions. In the first place, echoes are obtained not only 
from the submarine but from its wake, from water 
disturbances caused by previously exploded charges, 
from the wakes of surface vessels, from the ocean 
floor (in shallow water), and occasionally from the 
surface of the water. A good sonar operator is not 
easily led into thinking that these spurious echoes 
originate from the submarine itself, but it is in- 
evitable that a certain amount of error and confusion 
creeps into range and bearing information. In addi- 
tion, an inexperienced sonar operator may easily 
mistake a false target for the submarine and hence 
bring about an attack which is wholly futile. An in- 
dication of the importance of such mistakes is given 
by Table 1 which shows the frequency of errors in 
practice ahead-thrown attacks made at sea on “tame” 
submarines. It will be noted that the percentage of 
attacks on false targets is considerable even when an 
actual submarine is known to be in the immediate 
vicinity. 


Table 1. Attacks on false contacts in practice attacks. 


Area 

Type 

attacks 

Number 

on 

submarine 

Number 

not on 

submarine 

Percentage 
not on 

submarine 

Bermuda 

Hedgehog 

64 

18 

22 

Guantanamo 

Mousetrap 

59 

10 

14 

Key West 

Hedgehog 

170 

40 

19 

Key West 

Mousetrap 

575 

259 

31 

New London 

Mousetrap 

32 

8 

20 

San Diego 

Hedgehog 

86 

13 

13 

San Diego 

Mousetrap 

162 

22 

12 

Total 


1148 

370 

24 


Even assuming the contact to be on the submarine 
(or on some wake disturbance near and moving with 
the submarine), errors in sonar data are by no means 
negligible. Figure S shows how errors in range and 
bearing may be introduced by the submarine’s wake, 
as an example. It has been estimated that the average 
overall bearing errors are about 2 degrees if BDI 
(bearing deviation indicator involving lobe compari- 
son for accurate bearings) is used and 4 degrees and 
5 degrees if cut-on bearing procedures are used. The 
probable range error under the same conditions has 
been taken as 1 1 yd. These estimates include a nor- 
mal amount of wake echo and other errors usual in 
sonar operation. These errors are, of course, depend- 
ent on sonar conditions and sea sSte. When sound 



conditions are bad, the sonar operator is somewhat 
more likely to make errors. Roll and pitch of the 
ship in high seas introduce errors for several reasons. 
Present sound gear is not stabilized, so that violent 
roll and pitch introduce errors in the bearing re- 
corded and also make it difficult for the operator to 
keep the projector trained on the target. In addition, 
operator efficiency is reduced when the operator is 
training the gear with one hand, holding on to the 
bulkhead with the other, and combatting seasickness 
at the same time. The importance of such factors can 
hardly be determined theoretically, but operationaj 
data on the overall effect of these variables will be 
presented in a later section. 

One of the most serious limitations on sonar infor- 
mation is the minimum range at which it can be 
obtained. Figure 4 illustrates the reasons for this 
minimum range with present United States sonar. 
Because of the limited depression angle of the sound 
beam, the submarine can pass under it. This causes 
the ship to conduct the final part of a stern-dropped 
attack after contact with the target has been lost. 
Table 2 gives the average range of lost contact in at- 
tacks by United States surface craft. The increase of 



Figure 4. Minimum range on a deep submarine. 


116 


ATTACKS BY SURFACE CRAFT 


Table 2 . Lost contact ranges. 


Period 

Average range at which 
sonar contact was lost 

July 1942-Dec 1942 

176 yd 

Jan 1943 -July 1943 

192 yd 

Aug 1943 -Feb 1944 

223 yd 

Mar 1944 -May 1945 

279 yd 


average range reflects the increase in average depth of 
submergence of U-boats when attacked. 

The maximum sonar range is also of importance, 
primarily in regaining contact for repeated attacks. 
If the maximum range is less than about a thousand 
yards, the attack is made difficult because the attack- 
ing ship cannot maneuver freely and remain in con- 
tact with the submarine. For greater ranges, how- 
ever, maximum range has little effect on the accuracy 
of attack. 

Sonar ranges and bearings, such as they are, must 
be used in endeavoring to place the explosives so that 
they will arrive at a point beneath the surface at the 
same time the center of the submarine arrives at that 
point. How is this placement to be made, and how 
much in error will it be? 

The most important factor in this problem is the 
“blind time,” defined as the time elapsed between 
reception of the last useful information concerning 
the submarine’s position and the arrival of the ex- 
plosives at the predetermined depth. In the case of 
ahead-thrown attacks the barrage is usually fired be- 
fore contact is lost, making the blind time simply the 
time of flight of the charges plus the time required 
for them to sink to the proper depth. For a stern- 
dropped attack the blind time is usually about a 
minute or more; for an ahead-thrown attack it may 
be as little as 15 sec. Since these are long enough for 
the submarine to move a considerable distance, it is 
necessary for the conning officer on the antisubma- 
rine ship to estimate this movement and allow for it 
in placing his barrage. Figure 5 shows the tracks of 
ship and submarine in a typical depth-charge attack 
and indicates the necessity for taking the blind time 
into consideration. In this case 50 seconds elapse be- 
tween loss of contact and explosion of the depth 
charges. It is necessary for the conning officer to track 
the target from time 0 sec to time 40 sec, determine 
its course and speed, either implicitly or explicitly, 
and then, on the basis of this information, determine 
where it will be at time 90 sec. 



Figure 5. Plot of typical attack (figures give time in 
seconds). 


In carrying out an attack the conning officer does 
not know the submarine’s motion beforehand but 
must infer it from sonar range and bearing data. 
Normally this tracking is done implicitly as ranges 
are plotted on the sound range recorder in such a way 
as to allow rapid determination of the rate of change 
of range. From this information the recorder com- 
putes the time to fire the barrage. To determine the 
course which should be steered in order to place the 
charges at the proper point, the conning officer ordi- 
narily observes the rate of change of bearing and 
applies lead angle according to simple rules. Fre- 
quently, however, the attack is carried out by watch- 
ing the plot of the attack furnished by the antisub- 
marine attack plotter. In this case a plot quite similar 
to that of Figure 5 is presented. The conning officer 
estimates the geographical position of the submarine 
at explosion time (90 sec in Figure 5) and steers his 
ship accordingly. Here tracking is quite explicit. 

Now, errors in placing the barrage of explosives in 
the attack are intimately related to blind time, 
method of tracking, and sonar errors. Figure 6 is a 
simplified illustration of this relationship. Suppose 
we have tracked the submarine from point A to point 
B, at which point we lose contact with it. Because of 
the sonar errors the submarine will probably lie 
within a distance e of point B at the time of lost con- 
tact, where e is the probable error’^ in location. Fur- 
thermore, because of these sonar errors the exact 


b The rule of combination of errors used in equation (1) is 
consistent with the normal usage of the term probable error, 
but Figures 6 and 7 are not. They should be considered as illus- 
trations only, not exact diagrams. 


efiESH^EWTTXL • 


FACTORS DETERMINING THE SUCCESS OF ATTACKS 


117 



course and speed of the submarine at point B are not 
known. Suppose that the probable error in course 
estimation is Aa and the probable error in speed 
estimation is A5. Now suppose that we estimate 
C to be the point at which the submarine and 
the barrage will arrive simultaneously, that is, the 
point of explosion of the charges. Then the blind 
time tc is the time required for the submarine to 
travel at its estimated speed from B to C. Because of 
the errors involved, the submarine is not likely to be 
at point C at the time the charges arrive there, but 
probably will be somewhere within the shaded area 
around C. The probable error in the attack (the 
typical distance between the center of the barrage 
and the center of the submarine at the time of ex- 
plosion) will be approximately proportional to the 
square root of the area. If the blind time were longer 
so that D was the estimated point of explosion, the 
shaded area would be larger, and hence the probable 
error of the attack would be larger. It is apparent 
from Figure 6 that the area within which the subma- 
rine lies is approximately proportional to the square 
of the blind time and, therefore, that the probable 
error in the attack is approximately proportional to 
the blind time. 

In Figure 6, however, we have assumed that dur- 
ing the blind time the submarine maintains the same 
course and speed it had at point B where contact was 
lost. In this case the error in the attack is called the 
approach error. Unfortunately, submarines can and 
do change their course and speed very considerably, 
giving rise to an evasion error as well. Figure 7 is. 



therefore, a more realistic picture of the area within 
which the submarine may lie after a blind time tc. 
This area, it will be noted, is made up of two parts, 
one of which is the approach area of Figure 6, the 
other an evasion area dependent on the turning circle 
of the submarine, its acceleration a and the blind time 
tc. This second area measures the submarine’s evasive 
capabilities and increases in size approximately as 
the cube of the blind time. The total shaded area in 
Figure 7 represents the total plan error in placing the 
barrage. If the three sources of error are assumed to 
be independent, the effective area in which the sub- 
marine may be is written as in equation (1), 

A = + ko^vt)^ -\- ks^vt)^ (1) 

on the assumption that errors in estimating subma- 
rine velocity are proportional to the velocity. A 
more complicated assumption would replace the 
term with other powers of v, not greatly altering the 
dependence of A on v. The k’s are constant, v is the 
submarine speed, and t is the blind time. To calcu- 
late the values of the ^’s from physical characteristics 
of the gear which is used is an involved process, and 
therefore we will merely consider them as empirical 
constants. 

Analysis of experimental data derived from prac- 
tice attacks at sea and on the attack teacher has 
shown that the distribution of attack errors is in most 
cases similar to a Gaussian distribution. For purposes 
of calculation, therefore, it is frequently assumed 
that the attack error distribution is, in fact, Gaussian. 


118 


ATTACKS BY SURFACE CRAFT 


Table 3. Errors in practice attacks. 


Example 

number 

Type of 
attack 

Reported 

by 

Method 
of attack 

Submarine 

course 

Submarine 

speed 

(knots) 

Average 
range 
of lost 

contact 

(yards) 

Sinking 

time 

(seconds) 

Average 

attack 

error 

(yards) 

1 

Stern-dropped 

ASDevLant 

Attack teacher 

Straight 

0 

200 

25 

55 

2 

Stern-dropped 

ASDevLant 

Attack teacher 

Straight 

3 

200 

25 

91 

3 

Stern-dropped 

ASDevLant 

Attack teacher 

Straight 

5 

200 

25 

99 

4 

Stern-dropped 

ASDevLant 

Attack teacher 

Straight 

7 

200 

25 

124 

5 

Stern-dropped 

ASDevLant 

Attack teacher 

Highly evasive 

5 

200 

25 

117 

6 

Stern-dropped 

ASDevLant 

Attack teacher 

Evasive 

7 

200 

25 

152 

7 

Stern-dropped 

ASDevLant 

At sea 

Evasive 

About 3 

Less than 100 

25 

55 

8 

Stern-dropped 

COCTLant 

At sea 

Highly evasive 

About 5 

Over 100 

20 

170 

9 

Ahead-thrown 

(Hedgehog) 

CIT 

At sea 

Evasive 

About 3 

Not lost 

12# 

41 


• Includes time of flight. 


Equation (1) indicates that the corresponding prob- 
able error would be given by 

£2 = q. (2) 

where E = radial probable error of attack, 

V = submarine speed, 
t =■ blind time, 

K^, K 2 , = empirical constants. 

Equation (2) must be considered as an approximate 
expression in which the coefficients vary widely ac- 
cording to the evasive capabilities of the submarine 
and the tracking capabilites of the antisubmarine 
ship. The important point is merely that the attack 
error increases rapidly with increasing blind time, so 
that the chief problem in improving the effectiveness 
of antisubmarine attack is that of reducing the blind 
time. 

The probable error E described above is a horizon- 
tal or plan error only. A vertical or depth error is also 
present in most attacks because of uncertainty as to 
the depth of the submarine. Very few United States 
ships have been fitted with depth-determining sonar, 
and consequently errors in estimation of depth have 
been large. A rough estimate is given by the range at 
which contact is lost, but it is not at all reliable. 
Means of reducing the depth error are, therefore, of 
very great importance. 

Having considered the various sources of attack 
error, we must now determine what the overall mag- 
nitude of the error E is under some typical condi- 
tions. It is not possible to determine attack errors 


from operational data, since we do not have suffi- 
ciently detailed data concerning attacks on enemy 
submarines. There is, however, a fairly considerable 
amount of data available from practice attacks where 
records are sufficiently complete. Two sources are 
available. One consists of the runs made on the attack 
teacher, which is a mechanical device for reproduc- 
ing the conditions of an attack at sea; the other, of 
practice attacks made at sea against friendly subma- 
rines. Table 3 presents representative data from both 
sources. In all these attacks the depth of the subma- 
rine was known, so that only plan errors are involved. 

The first conclusion which can be drawn from 
Table 3 is that the attack errors increase with in- 
creasing submarine speed, other things being equal. 
This is shown by comparison of examples 1, 2, 3, and 
4. There are a number of reasons for this increase. 
The distance traveled by the submarine in the blind 
time is increased by higher speed, so that the shaded 
areas shown in Figures 6 and 7 are larger for higher 
speeds. In addition the average blind time is in- 
creased because the majority of attacks end with the 
submarine heading away from the ship, in which case 
high submarine speed results in a low rate of closing 
the range. Finally, attack on a high-speed submarine 
may require somewhat more difficult maneuvering by 
the attacking ship. 

Examples 5 and 6 show somewhat greater errors 
than do examples 3 and 4 because of the submarine’s 
evasive maneuvers, but the difference is not so large 
as might be expected. In these attacks however, the 
nonevasive attacks were mixed with the evasive ones 
so that the conning officer did not know ahead of 
time whether the submarine would evade or not. In 


FACTORS DETERMINING THE SUCCESS OF ATTACKS 


119 


such a case erroneous indications of evasive maneu- 
vers are frequently acted upon and the charges 
dropped in the evasion area of Figure 7 rather than 
in the approach area. Thus an evasive error exists, in 
effect, whether the target actually evades or not, as 
long as it has evasive capabilities which the attacker 
thinks it might use. The errors given in examples 1 
to 6 can, however, be well represented by an equation 
of the same type as (2), namely, 

E = V 602 -f 240^2 H- 22?^^ . (3) 

This is done in Figure 8, and it is observed that the 
agreement is good. 

Examples 7 and 8 show the large effect of training 
on the accuracy of attack. The short lost contact 
ranges and low speed involved in example 7 would 
lead one to predict an error of about 75 yd on the 
basis of the curves in Figure 8. The small observed 
error, 55 yd, probably indicates exceptionally high 
skill on the part of this ASDevLant team. In the case 
of example 8, the expected error would be about 1 15 
yd for a 5-knot submarine, but the actual probable 
error was 170 yd. This effect might be expected, since 
crews in the training at COTCLant were probably a 
good deal less skillful than the ASDevLant teams, 
because of less training and experience. 

Of particular importance is the figure for ahead- 
thrown attacks in example 9. The error given, 41 yd, 



Figure 8. Probable attack errors (Table 3, examples 
1 to 6). 


is less than that for stern-dropped attacks on a sta- 
tionary submarine in example 1. It is reasonable to 
conclude that the errors involved are those of locat- 
ing the submarine-the term ATi-and that the blind 
time for these ahead-thrown attacks is short enough 
largely to eliminate submarine evasion error and pre- 
diction error (the and terms). 

So much for the attack errors. We are not inter- 
ested in them for their own sake, but only for their 
effect on the probability of success in an attack. The 
next step is to determine the probability of a barrage 
launched in an attack proving lethal to the subma- 
rine. In order to calculate this probability of lethality 
or “effectiveness,” both the attack errors and the 
characteristics of the barrage itself must be taken into 
account. 


■ Weapon Lethality 

The method of taking the characteristics of the 
barrage into account can best be made clear by an 
example. Suppose we are dealing with a depth- 
charge barrage such as the one shown in Figure 9. 
If a depth charge explodes immediately alongside of 
the submarine, it will undoubtedly make a large hole 
in the pressure hull and almost surely sink it. If the 
depth charge is many miles away, it will cause no 
damage. The transition between these two situations 
is probably a gradual one with a considerable region 
in which an exploding depth charge may sometimes 
cause the submarine either to sink or to surface and 
may sometimes fail to do so, depending on the 
strength of the particular submarine and on the 
morale and skill of its crew. # 



Figure 9. Seven-charge depth-charge pattern (hypothetical). 


120 


ATTACKS BY SURFACE CRAFT 


30 


S20 

z 

CO 

5 

o 

< 

cr 

X 

t 10 


0 

Figure 10. Comparison of lethal radii of Torpex 2 and 

TNT (78-in. HTS hull). 

Experimental evidence suggests that the pressure 
hull will be split if the charge explodes within a cer- 
tain lethal radius of it. The lethal radius depends on 
the weight and type of explosive and on hull thick- 
ness, but its exact determination is difficult. The 
ideal method of determination would involve actual 
tests against enemy submarines, but this is rarely 
possible and most tests are made on models. In Fig- 
ure 10 curves showing the lethal radius for TNT and 
Torpex as a function of charge weight are given for a 
7/8-in. HTS hull. 

Although these curves cannot be taken as giving 
exact lethal ranges, it is believed that they give a good 
indication of effectiveness against the types of sub- 
marines encountered in World War II. In order to 
simplify calculations it is usually assumed that all 
charges exploding within a fixed lethal (or surfacing) 
radius cause lethal (or surfacing) damage and that no 
others are effective. For any given position of an ex- 
ploding depth charge there is, then, a “commanded 
volume” which has the property that any submarine 
whose center lies in the commanded volume is sunk, 
but any other submarine is unaffected. 

Calculation of Barrage Lethality 

The actual method of using the commanded vol- 
ume to calculate the effectiveness of a barrage fol- 
lows. First, a three-dimensional outline of the pres- 




Figure 11. Commanded volume for hypothetical barrage. 


sure hull is drawn around each charge, centered at 
the charge’s explosion point and oriented at the ap- 
propriate target angle (for example, 150 degrees). An 
envelope is drawn around these outlines so that it is 
everywhere 21 ft from them (for lethal radius of 21 
ft). The results of this construction are shown in 
Figure 11. Now if the center of the submarine lies 
within these commanded volumes of Figure 11, it 
will be sunk. Therefore to determine the lethal prob- 
ability of the barrage it is only necessary to determine 
the probability that the center of the submarine will 
lie within the commanded volume. This probability 
is determined by the distribution of attack errors, 
denoted by p(x, y, z), defined so that p(x, y, z)dxdydz 
is the probability that the center of the submarine 
will be in position (x, y, z) relative to the center of the 
barrage at the time the charges explode. The proba- 
bility P that the center of the submarine will lie in 
the command volume is 


j j j p{x,y,z) dxdydz. 


P = 

Commanded volume 


( 4 ) 


The integration indicated in equation (4) is ordi- 
narily carried out by graphical methods, working 
first with the plan errors, then with errors in depth. 
Since the commanded volume varies with orientation 
of the submarine, the process must be carried out for 
a number of target angles in order to obtain an 
average effectiveness for the barrage. The overall 
conclusion, however, is obvious— P increases with in- 
crease in the commanded volume and decreases as the 
attack errors increase. 

7 he commanded volume depends, of course, on the 
type of ordnance employed. Suppose that a barrage of 
contact-fuzed charges, rather than depth charges, had 


121 


FACTORS DETERMINING THE SUCCESS OF ATTACKS 


been considered in the above discussion. The com- 
manded volumes of Figure 11 would then consist of 
cylinders having a cross section approximately equal 
to that of the submarine‘s and extending from the 
arming depth down to the floor of the ocean. In this 
case the commanded volume would be much larger 
than for depth charges. If depth errors are large, that 
is, if p{x,y,z) has an appreciable value over some 
two or three hundred ft in depth, this additional 
volume would result in a larger value of F— a more 
effective barrage for the contact charges. If, on the 
other hand, the depth errors were limited to 30 ft or 
so, the greater concentration of the depth-charge 
commanded volume within this region due to greater 
lethal radius will cause the value of P to be larger for 
the depth-charge case than for the contact-charge bar- 
rage. In general, depth errors are very large with 
present United States equipment so that the addi- 
tional commanded volume of contact charges has 
been a point very much in their favor. Like contact 
charges, proximity charges cover a wide range of 
depth. Their commanded volumes are cylinders of 
cross section approximately equal to that of a depth 
charge. This being so, the amount of commanded 
volume of proximity charges lying within the desired 
depth range is always as great as, or greater than, 
that of depth charges. Charges equipped with prox- 
imity fuzes are, therefore, as effective as similar 
charges equipped with depth pistols when depth 
errors are small and many times more effective when 
depth errors are large. 

c A slight correction must be made to take account of charges 
that either hit glancing blows on the sides of the submarine and 
fail to explode or hit and explode in some position too far from 
the pressure hull to be lethal. 


Figures for the effectiveness of various types of 
barrages are presented in Table 4. The theoretical 
advantage of ahead-thrown weapons due to decreased 
blind time (and greater commanded volume for 
Hedgehog and Mousetrap) is clearly shown. At first 
thought one might expect the Squid to be much less 
effective than Hedgehog because it employs depth 
charges rather than contact-fuzed charges, thereby 
commanding a much smaller volume. Squid is used, 
however, in conjunction with depth-determining 
gear which lowers the probable depth error to a 
point where the large lethal radius of the Squid 
largely makes up for the Hedgehog’s ability to cover 
a large range of depths. In addition, the higher sink- 
ing speed of the Squid projectile gives a somewhat 
shorter blind time. For very deep submarines this 
becomes important and Squid is considerably the 
more effective weapon. It has the extra advantage 
that nonlethal barrages may still bring the subma- 
rine to the surface where it can be sunk by other 
weapons. 

11.2.3 Calculation of Probability of 
Success per Incident 

Up to this point only single attacks have been dis- 
cussed. Usually, however, an action against a sub- 
marine consists of a number of attacks which are 
grouped together as an incident for purposes of as- 
sessment. Unfortunately for the antisubmarine team, 
it cannot always deliver as many attacks as it may 
wish. In the first place, contact is usually lost for the 
reason illustrated in Figure 4. Contact may also be 
lost during an attack as a result of water disturbances 
of one kind or another. In the second place, there 


Table 4. Theoretical effectiveness of antisubmarine barrages. 



Submarine depth 




Probable 

Weapon type 


No. of 

Lethal radius 

effectiveness 

assumption 

Fuze 

charges 

(feet) 

(per cent) 

Side-stern-launched 

Equally likely from 

Depth pistol 

9 

21 

6 

depth charges 

100-300 ft depth 



Side-stern-launched 

Equally likely from 

Influence pistol 

9 

21 

24 

proximity charges 

100-300 ft depth 


Hedgehog (Mk 10) 

Equally likely from 

Contact 

24 

Contact 

28 


100-300 ft depth 



Mousetrap (Mk 22) 

Equally likely from 

Contact 

16 

Contact 

17 

Squid 

100-300 ft depth 

At 200 ft depth with 

Depth pistol 

3 

16 

21 


30-ft standard er- 

or 

or 


ror on account of 


6 


26 


depth -determin- 
ing feature 






122 


ATTACKS BY SURFACE CRAFT 


is never a 100-per cent probability of regaining con- 
tact once it has been lost. The formation of wakes, 
knuckles, and explosion disturbances often causes 
contact to be lost permanently after a small number 
of attacks. A well-trained team working in good sonar 
conditions will not experience great difficulty in re- 
gaining contact, but a poor team working under 
poor conditions will find it almost impossible to do so. 
The theoretical probability of success in an incident 
is thus a function both of the probability of success in 
a single attack and of the probability of regaining 
contact after the attack. On the assumption that the 
probability of regaining contact after an attack is a 
constant, independent of the number of attacks pre- 
viously made, the following equation can be written. 

Pj = Pa+ C(1 - P„) Pa + C^\ - PaYPa 


• . . +C”(1-P«)«P, 


1 - C-(\ - PaY 
M - C (1 - P„) ’ 


( 5 ) 


where Pj = probability of success per incident. 
Pa = probability of success per attack, 

C = probability of regaining contact, 
n = total number of attacks which can 
be delivered without exhausting 
ordnance. 


Operational data indicate values of C varying from 
0.50 to 0.90, depending on the number of ships pres- 
ent, period considered, and other factors. Table 5 
presents some typical values. 


Table 5. Probability of regaining contact. 




Coordinated group 


Single ship 

of ships 

Jan 1943-July 1943 

0.54 

About 0.8 

Aug 1943 -Feb 1944 

0.68 

About 0.9 


Since the probability of success per incident is 
much improved by an increase in probability of re- 
gaining contact, this is a strong argument for the use 
of coordinated groups of ships.*^ 

<1 The interpretation of these figures is open to some ques- 
tion, however, because incidents have normally been classed as 
coordinated only when several ships actually attacked the sub- 
marine. Cases in which several ships were on hand but only one 
released depth charges are not usually counted as coordinated. 
This type of selection introduces a bias such that coordinated 
incidents may be credited with values of C (and of lethality) 
which are higher than those actually obtained in operations. 


Although this discussion has presented by no 
means all details of attack theory, the main ideas 
have been mentioned. Accordingly, it is now desir- 
able to consider operational data which can be com- 
pared with the theoretical predictions. 

11 3 OPERATIONAL DATA ON 

EFFECTIVENESS OF ATTACKS 

The data available concerning attacks on enemy 
submarines are not sufficiently complete to enable 
one to reconstruct the details of each attack. For each 
incident (which may involve several attacks) certain 
basic information can be obtained as to the condi- 
tions under which attacks were made, the weapons 
used, the ships involved, and the resulting success as 
expressed in the assessment of the attack.® Most 
analyses of operational data therefore consist of 
breakdowns to determine the effect of changes in 
the conditions or nature of attack on the success as 
evidenced by the assessment. Some figures of this 
type will now be presented which are typical of the 
results obtained from operational data. 

11.3.1 Factors Influencing Attack Errors 

As was pointed out in the previous section, two 
overall factors determine the probability of success 
in an antisubmarine attack: the attack errors and the 
weapon effectiveness. One of the variables which is of 
importance in determining accuracy is the state of 
the ocean with respect to sound transmission. If 
sound conditions are bad, the overall effectiveness of 
the ships will be reduced. Data are presented in 
Table 6 which show that such is indeed the case.^ 
These figures, it should be noted, are given on a “per 
incident” basis. The effectiveness of an incident de- 
pends both on the probability of success in a single 
barrage and on the number of barrages that can be 
dropped in an incident before contact is lost. 


e During the course of World War II, this assessment was 
based on visible evidence of damage, survivors from the U-boat, 
if any, and supporting intelligence. After the German surrender, 
however, captured documents have become available to supple- 
ment this information, and assessments have been revised. The 
data presented here are based on the earlier wartime assess- 
ments. See the Appendix and Chapter 8 for further discussion 
of this question. 

f The figures given are for United States attacks from July 
1942 to July 1943 assessed A to G, for which the information 
necessary to estimate sound conditions from oceanographic con- 
siderations was given. 


OPERATIONAL DATA ON EFFECTIVENESS OF ATTACKS 


123 


Table 6. Effect of sonar conditions on attack effectiveness. 



Number of incidents 

■ 




Percentage of 

Sound 

Assessed 

Assessed 

damaging 

conditions 

A-G 

A-E 

incidents 

Good-Fair 

120 

27 

22 

Poor- Bad 

74 

5 

7 


Through the combination of these effects, good 
sound conditions lead to a larger fraction of sub- 
marines damaged or sunk per submarine encoun- 
tered. 

Another source of error and difficulty in localizing 
the submarine with sonar is the roll and pitch of the 
ship. Present sound projectors are not stabilized, so 
that if the ship rolls and pitches violently, the oper- 
ator has difficulty in keeping the projector pointed at 
the target. Much information is lost in this manner, 
and, furthermore, rather serious errors may be noted 
in the bearings if they are taken at one or the other 
extreme of the ship’s roll. In addition, general oper- 
ator efficiency is reduced under such conditions. The 
figures given in Table 7 show that such an effect ap- 
parently becomes important in seas classed as rough 
or higher. (The numbers are too small to give definite 
proof.) Moderate seas show no deleterious effect, 
however, possibly because smooth seas are likely to be 
accompanied by thermal gradients and layering 
which cause poor sound transmission. 

Table 7. Effect of sea state on attack effectiveness. 

(U. S. attacks from July 1942-Ji*ly 1943.) 


Number of incidents „ , . 

Percentage of 

Assessed Assessed damaging 
State of sea A-G A-E incidents 


Calm, smooth, slight 

116 

20 

17 

Moderate 

50 

10 

20 

Rough and higher 

13 

1 

8 


The attack errors depend on many things besides 
oceanographic conditions. The type of ship and 
sound gear involved in the attack have a great deal 
to do with it, as do the experience and skill of the at- 
tackers, the depth, speed, and evasive maneuvers of 
the submarine. Concerning the last we have no infor- 
mation since we do not know what the enemy sub- 
marine really did in any of the attacks. The type of 
sound gear involved is undoubtedly of importance, 
but United States experience involved only one gen- 


eral type of sonar gear. Even variations such as use of 
BDI would not be expected to result in a major in- 
crease in effectiveness such as would be clearly evi- 
dent in operational data. A considerable number of 
Japanese attacks were, however, made using listening 
gear only. Comparison of their effectiveness as esti- 
mated from United States submarine experience with 
that of echo-ranging attacks is made in Table 8. The 
difference between attacks of the two types is very 
striking and must be largely due to the superior 
accuracy of echo-ranging gear. The difference be- 
tween United States and Japanese echo-ranging at- 
tacks is probably largely due to differences in skill of 
personnel involved, though United States gear was 
undoubtedly the better of the two. 


Table 8. Effectiveness of listening and echo-ranging 
attacks. 



No serious 
damage 
(per cent) 

Major 
damage 
(per cent) 

Submarines 
sunk 
(per cent) 

Jap attacks on U. S. subs 
(July 1, 1943-March 31, 
1944) 

Listening 

99 


0 

Echo-ranging 

87 

12 

1 

U. S. attacks on U-boats 
using echo ranging 
1943 

85 

10 

5 

1944 

65 

5 

30 


The effect of training and experience on the part 
of the attack team is one which was of extreme prac- 
tical importance in the Battle of the Atlantic. Per- 
haps the best example of improvement with experi- 
ence is a set of figures on the success of Hedgehog at- 
tacks compiled by the British. The data are given in 
Figure 12. The rise from an effectiveness of 7.5 per 
cent per attack in 1 948 to over 20 per cent in late 1944 
and 1945 must be ascribed largely to training, since 
there was no radical change in the type of sound gear 
used, nor in the enemy’s evasive tactics. The values in 
Figure 12 are given on a per attack basis,g unlike 
those in Tables 6 and 7, which are on a per incident 
basis. Consequently, any changes in the number of at- 
tacks made per incident should not affect these 
values. The individual attacks made were undoubt- 
edly more accurate in the recent periods than they 

s In the analysis, attacks thought to have been made on the 
U-boat’s wake or made after a kill had already been assured are 
not counted. Hence the figures are a fairly pure measure of 
weapon effectiveness. 




124 


ATTACKS BY SURFACE CRAFT 



Figure 12. Success of British Hedgehog attacks. 

were when the weapon was new and crews inexperi- 
enced in its use. 

It is evident that the points plotted in Figure 12 
indicate a marked rise in effectiveness to a value of 
about 20-30 per cent, in accordance with the theoret- 
ical figures in Table 4. In the first months of Hedge- 
hog use, its results were very disappointing, since 
earliest theoretical predictions were more optimistic 
than 28 per cent. After the weapon had been in use 
for a year or so, however, it was used much more 
effectively, and theoretical studies were made some- 
what more conservative as a result of a better under- 
standing of the problems involved, so that the theo- 
retical predictions and the results now agree. 

The effect of experience and training can also be 
illustrated by the results obtained by United States 
crews in depth-charge attacks during the early years 
of the war. Figure 13 shows two curves— an effective- 
ness per charge and an effectiveness per incident. The 
increase in probability of success per incident is ob- 
viously greater than the increase per depth charge 
dropped. The latter measures the increase in attack 
accuracy, whereas the overall figure also depends on 
the number of charges dropped per incident. In Fig- 
ure 13B the theoretical effectiveness per depth 
charge is given for comparison with results achieved. 
In the early period there was a considerable discrep- 
ancy, but the agreement became fairly good in later 
periods. The even more abrupt rise in Figure 13A 
indicates that larger barrages, greater skill in regain- 
ing contact, and more frequent coordinated attacks 
contributed a great deal to increase the overall effec- 
tiveness of incidents. 



li. UJ 

oo 

^1 

ijJO 

Uq. 
O. tkJ 

©2 
tr — 

Q.CO 



B 


THEOR 
TABLE 4 

ETICAL VALUE 

, FROM 




0 1942 1943 1944 

Figure 13. Success of U. S. depth-charge attacks. 


11.3.2 Comparative Effectiveness 
of Weapons 

So much for the influence of factors having to do 
with attack accuracy. The weapon used is also of im- 
portance, and operational data can be used to show 
the relative merits of different types of ordnance. 
The most widespread innovation in the course of 
World War II was the introduction of Hedgehog. A 
comparison between Figures 12 and 13 suggests that 
operational data do bear out the theoretical value of 
the Hedgehog attack. A direct comparison is pre- 
sented in Table 9. There is a training factor which 
must be kept in mind. During the early periods 
Hedgehog was not used in such a way as to realize its 
full effectiveness. 


T 


OPERATIONAL DATA ON EFFECTIVENESS OF ATTACKS 


125 


Table 9. Comparison of Hedgehog, depth charge, and 
Squid. (On a per barrage basis.*) 


Period 

Nation- 

ality 

Depth 

charge 

Hedgehog 

Squid 

1st half 1943 

British 

5.4% 



2nd half 1943 

British 

4.0% 

7.5% 


1st half 1944 

British 

6.4% 

15.4% 


2nd half 1944 

1st quarter 

British 

5-1% 

28.1% 

18.2% (single) 
33.3% (double) 

1945 

Aug 1942- 

British 

All 

7% 

23% 

62% 

June 1944 

Mar 1944- 

Alliedf 

4.0% 

8.0% 


May 1945 

U. S. 

4.5% 

9.9% 



* As mentioned in Section 11.3.1, British studies discard attacks not 
thought to be potentially effective and therefore give higher figures 
than those based on all attacks including some on wakes, bubbles, etc. 

t Based on only those incidents in which Hedgehog was used for at 
least one attack. 


In evaluating the depth charge versus Hedgehog 
comparison, it must be kept in mind that Hedgehog 
attacks may, on the whole, have been made by better- 
trained ships, in better sound conditions, or on shal- 
lower submarines, since Hedgehog is not to be used 
under unfavorable conditions. In the figures quoted 
for August 1942 to June 1944, however, only depth- 
charge attacks made in incidents which involved 
Hedgehog are counted. 

In these cases the same ships are involved for both 
weapons and the general conditions of attack are the 
same. The superiority of Hedgehog is again demon- 
strated, so that it may be concluded that the superi- 
ority is inherent in the weapon. 

The figures on Squid success are based on a very 
small number of attacks and cannot be considered 
conclusive. They are, however, even better than the 
theoretical predictions, as expressed in Table 4, con- 
firming the high effectiveness expected of Squid. 

As an overall conclusion on the relationship be- 
tween theoretical and operational values for the 
effectiveness of various types of ordnance, one can 
say that theory gives a correct picture of the relative 
merits of the various types and that it also gives a 
reasonably accurate picture of their absolute values. 
In other words, theoretical studies provide a basis for 
evaluating the state of training of antisubmarine 
vessels by furnishing a standard to be reached and 
also indicate the direction for most profitable devel- 
opment of antisubmarine ordnance. 


Figure 14. Success of incidents as a function of number 
of attacks (U. S. craft, January 1943-February 1944). 


It has been implied throughout that the proba- 
bility of success in an incident is strongly infiuenced 
by the number of attacks made: the more attacks, the 
greater the chances of sinking a submarine. Opera- 
tional results prove that this is indeed the case. Fig- 
ure 14 shows the relationship between success and 
number of attacks. It is evident that the percentage 
of damaging incidents increases steadily as the num- 
ber of attacks per incident increases, corresponding 
to about 5 per cent damaged in each attack, which 
is in accordance with expectations. The percentage 
of lethal incidents also rises, a fact of considerable 
importance. It might be expected that the mere exist- 
ence of a fourth attack, for example, would mean 
that the previous three had not been lethal, and the 
fraction of submarines sunk in cases where four at- 
tacks were made would simply measure the proba- 

Table 10. Coordinated versus independent attacks. 


Inde- Coordi- 
pendent nated 


U. S. attacks, Atlantic and Mediterranean, 

Jan 1943 -Feb 1944 

Number of incidents (A-G, -f JS*) 176 18 

Number assessed A or B 9 3 

Per cent successful 5 17 

U. S. attacks, Atlantic and Mediterranean, 

March 1944 -May 1945 

Number of incidents (A-G, -f- JS*) 41 38 

Number assessed A or B 5 2I 

Per cent successful 12 55 

U. S. attacks. Pacific, December 1941- 
April 1944 

Number of incidents (A-G, -f JS*) 181 29 

Number assessed A or B 16 6 

Per cent successful 9 2I 


* Incidents are assessed JS when a submarine is believed to have 

been present but complete information on the incident was not yet 
available at the time of study. 


40 

30 

20 

10 









PER 

CENT D 

AMAGEO 












0 ^ 

PER CE 

1 

NT LET 
1 1 

c 

HAL 

1 


2 3 4 

NUMBER OF ATTACKS 


126 


ATTACKS BY SURFACE CRAFT 


bility of sinking in the fourth attack. Figure 14, in- 
dicates, however, that lethal damage may accumu- 
late as a result of a succession of attacks. When a num- 
ber of attacks have been made, the next is more likely 
to prove lethal than it would otherwise. Figure 14 
does more than confirm the importance of persist- 
ence which was demonstrated by equation (5). This 
equation was based on the assumption that each at- 
tack had a fixed chance of success, whereas the opera- 
tional results suggest strongly that the chance gets 
better with each succeeding attack. It may be con- 
cluded that regaining contact for persistent and re- 
peated attacks is of the utmost importance. 

Employment of several ships in coordinated hunt 
does much to assure that contact will be regained a 
large number of times, as shown in Table 5. Corres- 
pondingly, coordinated incidents have a high proba- 
bility of success. Some typical comparisons between 


independent and coordinated incidents are given in 
Table 10. The coordinated incidents are consistently 
at least two or three times as effective as the inde- 
pendent.^ 

The overall conclusions concerning antisubmarine 
attacks are simple. For good effectiveness three things 
are required: (1) good attack accuracy through 
proper design of sound gear and ordnance and train- 
ing of personnel, (2) good weapon effectiveness 
through a large commanded volume, and (3) per- 
sistent and repeated attacks with good ability to hold 
and regain contact. 


h This effect may be somewhat exaggerated by the method of 
designating coordinated incidents. If two ships make attacks a 
few hours apart on what was probably the same submarine, the 
actions involved will be likely to be considered a single coordi- 
nated incident if damage is done, two independent incidents if 
there is no damage. 


C( 


Chapter 12 

ATTACKS BY AIRCRAFT 


*2 * GENERAL STATEMENT 

OF PROBLEM 

T he general theory of aircraft attacks can be ap- 
proached along the lines used in considering 
surface craft attacks. Although the details of the two 
subjects differ widely, many of the basic theoretical 
ideas developed in the previous chapter are applic- 
able in the present one. Furthermore, the general 
type of operational data available and the system 
used in assessing attacks are the same. As before, we 
shall first give a brief overall description of the prob- 
lem and then proceed to detailed considerations. In 
both cases, a priori and a posteriori aspects will be 
treated. 

Aircraft are greatly superior to surface craft in lo- 
cating submarines on the surface, but their effective- 
ness for underwater search and tracking is very lim- 
ited.a As a result, aircraft are primarily of value in 
attacking submarines sighted on the surface. Many 
attacks will actually be delivered while the subma- 
rine is still fully or partly surfaced, but the rapidity 
with which a submarine can crash dive as the aircraft 
closes to attack means that attacks shortly after sub- 
mergence must also be considered.*^ 

When an aircraft has made contact with a surfaced 
submarine (either visually or by radar), it must next 
get into a favorable position to make an attack. Since 
the target is small, it is necessary to get down to a low 
altitude for maximum accuracy. The target will, 
however, usually submerge on sighting the aircraft in 
order to escape attack, and therefore the approach 
must be made in such a way as to obtain the maxi- 
mum element of surprise and limit, as much as pos- 
sible, the degree to which the submarine can sub- 
merge prior to attack. During the approach the 
course and speed of the submarine must be estimated 
so that allowance can be made for target motion. 
When the proper position has been reached, the air- 
craft releases its weapons. These may be either 

a See Chapter 13 for a further discussion of this point, 
b Use of Schnorchel by the submarine will, of course, greatly 
reduce the searching effectiveness of the aircraft but will not 
change the basic attack problem except to the extent that it 
increases the average degree of submergence of submarines 
when attacked. 



rockets or bombs. As in the case of surface craft at- 
tacks, the problem is to release the weapons so that 
they will reach a point beneath the surface at the 
same time the submarine reaches that point. (See Fig- 
ure 1.) From a knowledge of the characteristics of the 
weapons and of the position of the submarine at the 
time of attack, it is possible to determine where the 
weapons should strike the water to be effective. The 
probability of success will then depend on the accu- 
racy with which the correct target position is esti- 
mated, on the errors made in placing the weapons in 
the desired position, and on the lethality of the 
weapons used. 

The above factors will be considered in detail in 
the balance of this chapter. We shall first give a theo- 
retical discussion and follow with a consideration of 
operational results. Since depth bombs have so far 
been the primary aircraft weapon against the sub- 
marine, operational experience is most extensive for 
this weapon. Certain other phases of operational re- 
sults cannot be discussed at this time because highly 
classified information is involved. Hence much of the 
following detailed discussion will be confined to the 
depth bomb and it should be considered as an ex- 
ample of methods of evaluaton rather than as a com- 
plete examination of the subject of aircraft attack 
weapons and tactics. 

12 2 THEORETICAL DISCUSSION OF 
FACTORS DETERMINING THE 
SUCCESS OF ATTACKS 

The factors determining the probability of success 
in an aircraft attack can be grouped in the same two 
general categories as were involved in surface craft 
attacks: attack errors and weapon lethality. Attack 
errors, in turn, may be subdivided as follows. 


127 


128 


ATTACKS BY AIRCRAFT 


1. Errors in estimating submarine position. 

2. Errors caused by variation in the behavior of 
individual missiles. 

3. Aiming errors. 

Attack Errors 

If weapons are released while the submarine is still 
visible, errors in estimating target position will be 
restricted to misestimates of target motion from the 
time of release to the time of impact or explosion 
(the “blind time”).*^ For rockets such movement of 
the submarine is so small that no allowance is ordi- 
narily made for it. For depth bombs, the time from 
release to explosion will be on the order of 5 sec or 
so, depending on exact conditions of attack, in which 
time a submarine can travel only about one-half its 
length. It is fairly easy to make allowance for such 
changes in target position and errors will be neg- 
ligible. 

On the other hand, if the submarine submerges 
completely before attack, there is a longer blind time 
and the uncertainty of submarine position will in- 
crease with increase in this blind time, in the manner 
discussed in Chapter 11. The effect of a long blind 
time will be especially serious in aircraft attacks since 
there is no information as to the target’s course and 
speed except visual estimation, and the barrage which 
can be dropped by aircraft is too small to cover radi- 



Figure 2. Submarine evasion area as a function of time. 


cal changes of speed and course on submerging. The 
expansion of the possible area in which the subma- 
rines may be at the end of a given number of seconds 
after submergence is shown in Figure 2. This is based 
on the turning characteristics of the 500-ton German 
U-boat and assumes that speed may vary from 3 to 7 
knots. It will be noticed that the possible area re- 
mains very small for the first 15 sec or so and then 
increases rapidly until, at the end of 1 minute, it is 
about 270,000 sq ft. 

From the above discussion, it is evident that attack 
errors due to misestimation of target position in- 
crease so rapidly with time after submergence that 
the probability of success quickly approaches zero. 
For Class A attacks, which are defined as those made 
on visible submarines or on submarines submerged 
less than 15 sec, the submarine’s position is quite 
well known. For Class B attacks, which are attacks on 
submarines submerged between 15 and 30 sec, the 
submarine’s position has become uncertain. For at- 
tacks made still later, the probable error in estimat- 
ing submarine position has become extremely large. 
A priori, therefore, we would expect a much greater 
degree of success in Class A attacks than in others, 
the probability diminishing rapidly for Class B and 
later attacks. An accurate, quantitative a priori evalu- 
ation is not available, but one based on operational 
experience will be given later to bear out the above 
qualitative conclusion. 

The obvious method of reducing errors of the type 
just discussed is to reduce blind time, that is, to make 
as many attacks as possible on visible submarines or, 
at least, on those within the Class A category. It is 
therefore important to employ tactics designed to 
achieve the greatest amount of surprise in the attack. 
Some of the factors involved are speed of aircraft, 
correct patrol altitude, approach from cloud cover, 
use of camouflage to avoid visual detection of air- 
craft, and use of countermeasures to search receivers 
to avoid detection of radar emissions.*^ It is also ad- 
vantageous to make attacks in locations where sub- 
marines are less alert, that is, to surprise them in 
areas where aircraft attack is not expected. In addi- 
tion to such measures, the number of favorable at- 
tacks can be increased by avoiding attacks which in- 
volve too great a blind time and which therefore 
have a negligible probability of success so that time 
and weapons may be conserved for possible future 


c Blind time has been defined in Chapter 11. 


d See Chapter 14. 


FACTORS DETERMINING THE SUCCESS OF ATTACKS 


129 


opportunities against Class A targets. The effect of 
all such measures in increasing the proportion of 
Class A attacks is somewhat difficult to evaluate a 
priori, but we shall show later, from operational ex- 
perience, the overall improvement which resulted 
from emphasis on the importance of prompt attacks 
involving the maximum element of surprise. 

The second class of attack errors mentioned above 
involved those due to variation in the behavior of 
individual missiles. In aiming a given weapon, it is 
necessary to assume a certain behavior after it leaves 
the aircraft. Variations from this normal will, of 
course, occur. Depth bombs, for example, will vary 
in their fall through air and in their underwater 
travel. There will also be a variation in the depth at 
which they explode. Similarly, rockets will vary in 
flight path and in underwater trajectory. Such devia- 
tion will, of course, decrease the accuracy of the at- 
tack. Errors of this type can only be reduced by im- 
proved design of weapons. Extensive practical tests 
and analyses of operational results will often prove 
of value in determining the effectiveness of improved 
design. 

As an illustration of the size of errors of the type 
just discussed, the following estimates for depth and 
contact bombs are quoted in Table 1. 


Table 1. Variation of individual missiles. 
(Aircraft speed 100-200 knots, altitude 50-200 ft.) 


Type of bomb 

Probable 
deviation 
along 
aircraft 
course (feet) 

Probable 
deviation 
perpendicular 
to aircraft 
course (feet) 

Round-nose U. S. depth bomb 

17 

17 

Flat-nose U. S. depth bomb 

7 

7 

Contact bomb (Hedgehog or 



Mousetrap) 

31/2 

0 


Considerations of target position and weapon 
characteristics determine the point on the water at 
which the weapons must strike to be effective. Errors 
in placing them in the desired position may be called 
aiming errors. This is the third subdivision of attack 
errors mentioned above. Aiming errors are com- 
monly measured with relation to the aircraft’s course. 
Errors along its course are called range errors, while 
those perpendicular to its course are called line 
errors. 

Line errors are caused by failure of the pilot to fly 
in a straight line directly over the aim point, while 



Figure 3. Geometry of horizontal bombing. 

range errors are due to release of bombs at the wrong 
moment, or, in the case of rockets, to improper alti- 
tude of the plane at the moment of firing. The size 
of these errors will depend on the conditions of at- 
tack, on the skill of the pilot and bombardier, and on 
the accuracy of the bomb or rocket sight used. 

As an example of aiming errors and methods of 
reducing them, consider the problem of delivering 
an attack with depth bombs. The pilot flies as nearly 
as possible straight across the aim point and thus 
controls the line error. The problem as far as range 
is concerned is, then, to determine the proper mo- 
ment for release of the bombs. For a horizontal at- 
tack, the geometry of the situation is simple. In Fig- 
ure 3, P represents the position of the plane at 
moment of release of the center bomb of the stick, A 
represents aim point, h is altitude, r slant range, and 
I horizontal range (in feet). The angle a is the angle 
between the line PA and the horizontal. Then, disre- 
garding air resistance, t he cen ter bomb will travel 
forward a distance V^/Whjg ft, where V is plane 
velocity in feet per second. Hence correct release will 
occur when / = FV2 h /g or r = Vh~ + 2h/g or 
when a has the value determined by the relationship 
tan a = h/l. Hence the aiming problem in range 
can be solved for a given altitude by selecting either 
the proper slant range, horizontal range, or angle 
between the horizontal and line from aircraft to aim 
point. Use of r is indicated for radar bombsights 
since slant range can be determined by radar. Use of 
the angle « is involved in using the reflector gun- 
sight which enables determination of this angle. In 
addition to the above means it is possible to utilize 
the rate at which the angle a is changing, as is done 
in the angular velocity bombsight. Whereas other 
methods are sensitive to correct determination of 
speed and altitude this latter method is relatively in- 
sensitive to errors in such factors. 

The aircraft will not necessarily make a level ap- 
proach to the target. On sighting the submarine the 
plane will normally be at a rather high search alti- 
tude and must lose altitude to make the attack. 


130 


ATTACKS BY AIRCRAFT 


Hence it is often natural for the plane to make the 
bombing run while still in a glide. In a glide attack 
the formulas for determining release point are some- 
what more complex than in horizontal bombing, but 
similar methods of controlling range errors can be 
used. There is also the possibility of using the plane’s 
motion in pulling out of a glide to release the bombs, 
a method known as toss-bombing. 

Commonly used during the recent war, because of 
the lack of suitable bombsights, was the so-called 
seaman’s eye method of bombing. This term is ap- 
plied to bombing without a sight in which the pilot 
releases bombs at the proper moment by instinct 
gained over long periods of practice. 

Aiming errors, for a given method of aiming, will 
vary widely with such factors as type of aircraft, de- 
gree of training and individual ability of pilots and 
bombardiers, conditions of attack, etc. It is not pos- 
sible, therefore, to quote figures of general applica- 
bility. The performance of a typical TBF squadron 
trained in glide bombing at ASDevLant gives an in- 
dication of the order of magnitude of such errors in 
training and of the effect of practice in reducing 
them. 

During a 3-week period, each pilot made about 
100 practice attacks on a towed target, using a glide 
angle of about 15 degrees and aiming by means of a 
reflector sight. The mean point of impact [MPI] for 
all attacks was 62 ft over in range and 8 ft right in 
line. Probable error about the MPI was 80 ft in range 
and 30 ft in line.® 

During the 3-week period the MPI in range de- 
creased from about 135 ft to 18 ft. The MPI in line 
did not improve. The probable error about the MPI 
decreased from 98 ft to 40 ft in range and from 38 
ft to 25 ft in line. It is evident, therefore, that train- 
ing brought the MPI effectively on the target and 
reduced dispersion about the target very noticeably. 
Improvement was still continuing after 100 practice 
attacks per pilot. 

After the 3-week training period in glide bombing 
the squadron spent a week in horizontal bombing by 
seaman’s eye. The MPI was 31 ft over in range and 1 
ft right in line, with probable errors about the MPI 


e It should be noted that line errors varied noticeably with 
angle between aircraft course and target course. The MPI was 
on target for track attacks and 10 or 20 ft right for beam attacks, 
according to whether approach was from port or starboard. 
Probable error about the MPI was about 15 ft for track attacks 
and 40 ft for beam attacks. 


of 66 ft in range and 14 ft in line. Since these results 
were obtained after the extensive practice in glide 
bombing, a direct comparison of overall results by 
the two bombing methods is not fair, but it can be 
judged that range errors in horizontal bombing were 
about 20 per cent greater than in glide bombing 
while line errors were only about half as great. 

The improvement possible by use of an accurate 
sight is suggested by the fact that tests at ASDevLant 
with the BARB (angular velocity) sight showed a 
probable error of only 16 ft in range. It was found 
that very little training was required. Similar im- 
provement in rocket accuracy by use of proper sights 
is indicated by ASDevLant tests. Using the reflector 
sight with prescribed sighting allowance, mean devia- 
tion of about 10 mils in range was achieved by the 
best trained squadrons, the RASP (automatic vector) 
sight gave 8 mils, and toss-rocketing, 6.3 mils. 

Operational errors usually proved considerably 
greater than those obtained in practice attacks. These 
will be discussed in the next section. 

The effect of the three types of errors discussed 
above on the success of an attack can only be deter- 
mined by considering such errors in connection with 
the lethality of a given weapon. We shall therefore 
next discuss weapon lethality and then illustrate the 
combination of attack errors and weapon lethality in 
determining a priori probabilities of success. 

12.2.2 Weapon Lethality 

From a general point of view, the concept of com- 
manded volume discussed under surface craft attacks 
is applicable to aircraft attacks. For example, if a 
stick of depth bombs is dropped, each bomb will 
have around it a commanded volume constructed by 
the method previously given. The probability of suc- 
cess will be given by equation (1). 

^^/// ( 1 ) 

Commanded volume 

where the function p{x,y,z) is the probability that the 
center of the submarine is at position x,y,z and is de- 
termined by the attack errors. The probability of a 
hit on the pressure hull by a salvo of rockets could be 
similarly determined; in this case the commanded 
volume of each rocket would be the solid generated 
by moving along the underwater trajectory of the 
rocket an area equal to the cross section presented by 


FACTORS DETERMINING THE SUCCESS OF ATTACKS 


131 



Figure 4. Commanded volume of antisubmarine rocket. 


the pressure hull for the given angle of attack.^ (See 
Figure 4.) 

Because of the effect of blind time on bombing 
errors, however, aircraft attacks must be made on 
surfaced or nearly surfaced submarines to be success- 
ful. It is convenient, therefore, to eliminate subma- 
rine depth as a variable in the problem and to make 
probability calculations on the basis of an assumed 
depth or a small range of equally probable depths. 
This enables us to replace the concept of com- 
manded volume by one of lethal area. 

This method may be illustrated by considering an 
attack against a German 500-ton U-boat by an air- 
craft dropping a stick of depth bombs of the type 
used in World War II. For such attacks, a fixed 
depth setting of 25 ft was ultimately adopted. This 
is approximately correct for the average Class A 
submarine if we consider all depths of the pressure 
hull’s center between 6I/2 ft (for surfaced U-boats) 
and 40 ft (for U-boats down 15 sec) as equally likely. 
Since bombs set for 25 ft actually exploded some- 
what deeper, we shall assume an effective depth set- 
ting of 30 ft for our illustration. (This is probably 
about correct for the best United States fuze devel- 
oped in World War II.) 

Considering, then, only Class A attacks made with 
this effective depth setting and assuming all depths 
between 61/2 ft and 40 ft as equally likely, the lethal 
area may be determined as follows. The average dis- 
tance from the center of the pressure hull measured 
perpendicular to the submarine’s keel within which 
a depth bomb must explode to be lethal is found 
from Figure 5. The bomb will be effective provided 
the center of the pressure hull lies within the shaded 
area of the diagram, that is, within a radius of the 
point of explosion equal to the lethal radius of the 
bomb plus the radius of the pressure hull. In the dia- 


f Angle of attack is angle between aircraft course and sub- 
marine course. 



Figure 5. Depth coverage. Depth bomb, lethal radius =: 
1714 ft. Average width covered = 40 ft. 


gram, a lethal radius oi\ 7 i /2 ft has been used; this is 
about correct for a TNT-filled, 350-lb United States 
depth bomb. From this diagram the average width 
for which such a depth bomb is effective can be easily 
determined; it comes out as about 40 ft. In other 
words the charge must be at a distance, measured 
perpendicular to the keel, of not over 20 ft from the 
center of the pressure hull. Consider next the dis- 
tance measured parallel to the submarine’s keel 
within which such a bomb must explode to be lethal. 
For the 500-ton U-boat, allowing for variation in 
diameter at the ends and also for the fact that the 
charge may be effective somewhat ahead or aft of the 
hull, it is found that a bomb with a 17i/2-ft lethal 
radius should be effective if it explodes not more than 
95 ft ahead or astern of the pressure hull’s center, 
based on the same considerations as to depth as were 
previously used. 

It follows, therefore, that for the kind of bomb and 
submarine considered there is a lethal area 190 ft 
long and 40 ft wide on the surface of the water with 
its center directly above the center of the pressure 
hull, as shown in Figure 6. Thus if a bomb explodes 
below this area at the assumed depth of 30 ft, it will 
sink the submarine.^ The probability that this will 
occur depends, of course, on the attack errors. 

Another point of view is to consider this lethal area 
as surrounding each bomb, with its long dimension 
in the direction of the U-boat’s keel. Then if the 
center of the submarine lies within the lethal area of 
any one of the bombs of the stick, the attack will be 


g This is not strictly true of course, since there are bound to 
be fluctuations; some bombs will explode under the lethal area 
and fail to destroy the target, while others somewhat outside 
this area will succeed. The assumption of a fixed lethal area can, 
however, give us the correct average expectancy for a large num- 
ber of cases. 


132 


ATTACKS BY AIRCRAFT 




40 ’ 

cr 

1 







Figure 6. Lethal area for depth hoiiib (TNT-filled 350-lh 
bomb). 


successful. As before, the probability of this occur- 
ring may be found by consideration of the attack 
errors. 

Similar methods of approach can be used for other 
types of bombs. The problem for rockets is also anal- 
ogous. Under given conditions of submarine depth 
and angle of attack, considerations of the underwater 
trajectory of the rocket during the period for which 
speed is adequate for penetration will indicate how 
far short of the target, in range, it can strike and still 
be lethal.*^ The aspect presented by the submarine 
will determine permissible variation in line. For ex- 
ample, in a beam attack on a surfaced submarine 
with a 15° glide angle, it has been estimated that a 
Model 5 rocket will be lethal if it strikes the water not 
more than 67 ft short of the submarine nor more than 
80 ft to either side— that is, under these conditions 
the lethal area is 67x160 ft, as shown in Figure 7. 



Figure 7. Lethal area for rocket against surfaced sub- 
marine (beam attack). 


Penetration of the pressure hull by a rocket may not always 
cause immediate sinking, but the resulting damage should nor- 
mally be sufficient to keep the submarine on the surface and 
permit follow-up attacks to sink it. Hence the term lethal may 
reasonably be used. 


12.2.3 Probability of Success 

We can illustrate the method of combining attack 
errors and weapon lethality to determine a priori 
probabilities of success by considering a stick of four 
depth bombs under the same assumptions as were 
used in our illustration of lethal area. For simplicity 
let us assume an attack made from directly on the 
beam and disregard individual dispersion of the 
bombs. We shall further assume that probable attack 
errors in range and line are 120 ft and 65 ft, respec- 
tively, and that these are normally distributed about 
the U-boat’s center.^ Under such assumptions it is 
evident that the most effective stick spacing is 40 ft, 
since with this spacing the lethal area of each bomb 
just touches that of each of the adjacent bombs and 
there are no gaps or overlaps in the total lethal area 
of the four bombs. We have, then, an effective lethal 
area for the whole stick of 160 ft along the aircraft’s 
track and 190 ft perpendicular to the aircraft’s track, 
as shown in Figure 8. If the center of the submarine 
lies within this total area the attack will be a success. 

Based on the assumed probable error of 120 ft in 
range, the curve showing the probability that the 
center of the submarine will be a given distance in 
range from the center of the stick is simply the nor- 
mal distribution curve shown in Figure 9. The prob- 
ability that the center of the submarine will be 
within 80 ft of the center of the stick in range is, 
therefore, the area under this curve from x = —80 to 
X = +80, namely, 0.35. 

A similar calculation of the probability that the 
center of the submarine will be within 95 ft of the 
center of the stick in line, based on the assumed prob- 



i These errors, considerably greater than those quoted earlier 
lor practice drops, arc still rather small for operational errors. 


FACTORS DETERMINING THE SUCCESS OF ATTACKS 


133 


Table 2. Characteristics of bombs. 


Type 

Explosive 

Nose 

Lethal 

radius 

Depth of 
explosion 

Dispersion 

Along 

aircraft I’erpendicular 

course to aircraft 

350-lb depth bomb 

J TNT 

{Flat 

I7iA ft 

30 ft 

7 ft 

7 ft 

(explosive 250 lb) 

(Round 

Ui/^ft 

30 ft 

17 ft 

17 ft 


1 Torpex 

Flat 

22 ft 

30 ft 

7 ft 

7 ft 

650-lb depth bomb 

1 TNT 

{Flat 

25 ft 

30 ft 

7 ft 

7 ft 

(explosive 450 lb) 
60-lb contact 

(Round 

25 ft 

30 ft 

17 ft 

17 ft 

(explosive 30 lb) 

TNT 

Flat 

Contact 

Contact 

3iAft 

0 


able error of 65 ft in line, gives 0.68. The actual prob- 
ability of success depends on the occurrence of both 
these events and is therefore 0.35 X 0.68 = 0.24; in 
other words, under the assumptions made the stick of 
bombs has a 24 per cent chance of killing the sub- 
marine. 

The calculation is considerably more complex if 
individual bomb dispersion is taken into account 
and if angles of attack intermediate between 0 de- 
gree and 90 degrees to the submarine’s course are 
considered. The above example, however, illustrates 
the basic theory involved. We shall next examine the 
results of some a priori calculations of this sort. 

Such calculations serve several purposes. They 
make it possible to determine optimum tactics for 
the use of a given weapon and answer such questions 
as: what is the best angle of attack, what is the best 
spacing for bombs in a stick, etc. More fundamen- 
tally, they make it possible to compare the expected 
effectiveness of different basic types of weapons and 
of different models of a given type. Such comparisons 
are of value in determining which weapons should be 


Y 



DISTANCE IN RANGE FROM CENTER OF STICK 

Figure 9. Probability that center of submarine will be a 
given distance in range from center of stick. 


used and often suggest profitable improvements in 
their design. A priori probability calculations may 
also be used to study the effect of accuracy in the use 
of a given weapon and show to what extent improve- 
ment in probability of success is possible through 
improvement in accuracy. Finally, such calculations 
predict what should be expected from operational 
results and are useful in evaluating such results. We 
shall illustrate the above points by considering the 
use of bombs in Class A attacks against the German 
500-ton U-boat with the same assumptions as to the 
depth of the pressure hull’s center as were used in 
discussing lethal area (i.e., all depths between 61/2 ft 
and 40 ft are considered equally likely). Results will 
be shown for the basic types of United States bombs 
used in World War II. Table 2 summarizes the as- 
sumptions made as to their characteristics. (Weight 
is given in round figures— individual types of bombs 
vary somewhat in weight.) 

Assumptions as to probable aiming errors (includ- 
ing probable submarine position error) are shown in 
Table 3 for beam attacks and lengthwise attacks. 
(Consistent values have been used for intermediate 
angles of attack.) 

On the above assumptions the effect of stick spac- 
ing on probability of success is illustrated in Figure 
10 for 350-lb, TNT-filled, round-nose depth bombs. 
On the basis of similar calculations for various angles 
of attack, the curves shown in Figure 11 can be ob- 
tained. It will be noted Irom Figure 1 1 that there is a 


Table 3. Assumed bombing errors for Class A attacks. 



Probable error 

Probable error 


in range 

in line 

Beam attack 

120 ft 

65 ft 

Lengthwise attack 

135 ft 

20 ft 


134 


ATTACKS BY AIRCRAFT 



STICK SPAdNC IN FCCT 


Figure 10. Probability of sinking as a function of stick 
spacing, 350 lb, round-nose depth bombs, TNT- filled. 


wide variation in optimum spacing for a two-bomb 
stick according to angle of attack and only small 
variation for other stick sizes. Figure 10, however, 
shows that the actual effect of stick spacing on prob- 
ability of success is almost negligible for a two-bomb 
stick and is greatest for the longer sticks. In other 
words, for cases where the optimum varies, the im- 
portance of using the optimum value is small, where- 
as for cases for which use of an optimum value is 
important nearly the same value is optimum for all 
angles of attack. It follows therefore that an overall 
value of about 75 ft for all angles is quite satisfactory. 



Figure 11. Optimum stick spacing as a function of angle 
of attack, 350-lb, TNT-filled, round-nose depth bombs. 



Figure 12. Probability of sinking as a function of angle 
of attack, with best stick spacing for each angle. 


This is an important conclusion, since varying the 
stick spacing for each attack would result in compli- 
cation and delay. It can also be seen from Figure 10 
that even for larger sticks the use of values deviating 
somewhat from the 75-ft value suggested will be 
quite acceptable. (Similar conclusions have been 
found for flat-nose bombs.) 

The effect of angle of attack on success is illus- 
trated in Figure 12 for the round-nose depth bombs 
just considered and for corresponding flat-nose 
bombs. It is apparent that angles along the subma- 
rine’s track give the best probability of success. The 
improvement possible by selecting this angle makes 
it worthwhile to do so wherever convenient. How- 
ever, the gain is not sufficient to warrant delay in 
attack for this purpose. 

The above results are merely illustrative since the 
effect of stick spacing and angle of attack will vary 
with the type of bomb considered and the assump- 
tions made in the calculations, but it is felt that the 
conclusions may safely be applied to low-level depth 
bomb attacks on Class A submarines. We shall now 
proceed to a comparison of the different types of 
bombs listed in Table 2, averaging results over all 
angles of attack and using the best stick spacing at 
each angle. 

Such a comparison is most instructive if made on 
a weight-for-weight basis, as has been done in prepar- 
ing the curves shown in Figure 13. 

One fact is immediately evident from these curves. 
There is a great gain in effectiveness with number of 
bombs dropped, regardless of type. This means that 
it is important to drop a sufficient number in the first 
attack to secure a high probability of success. With- 
holding bombs is not warranted unless there is a very 
good chance of making a second Class A attack. For 
example, if the chance of a second attack is less than 
25 per cent, as many as ten 350-lb bombs are justi- 
fiably expended on the first attack. 




FACTORS DETERMINING THE SUCCESS OF ATTACKS 


135 


< -J 

UJ CO 

o ^ 
P o 

Z 3 
UJ cn 
o 
a: 

UJ 

o. 


100 

80 

60 

40 

20 

0 











B 

\ 




( 








-D 





— E 




0 1000 2000 3000 

WEIGHT OF BOMB IN POUNDS 


Figure 13. Probability of sinking as a function of weight 
of bombs in stick for various types of bombs. Averaged 
over all angles of attack with optimum stick spacing at 
each angle. /I— 350-lb Torpex flat-nose; i?— 350-lb TNT 
flat-nose: C— contact; D— 350-lb TNT round-nose; E— 
650-lb TNT flat-nose; F-650-lb TNT round-nose. 


A comparison of the results according to type of 
bomb illustrates several important points. These 
may be summarized as follows. 

1. A comparison of the curves for round-nose 
bombs with the corresponding curves for flat-nose 
bombs indicates the marked improvement achieved 
by decreasing the dispersion in underwater trajec- 
tories of individual bombs from 17 ft to 7 ft through 
proper design of the bomb. 

2. A comparison of the curves for 350-lb bombs 
with those for 650-lb bombs shows that on a weight- 
for-weight basis the smaller bombs should be mark- 
edly superior for all sizes of bomb stick. Although 
the 350-lb depth bomb is the lightest used by the 
United States Navy, it is not necessarily optimum. A 
study, under the same assumptions, of the British 265- 
lb bomb (weight of Torpex, 193 lb) showed somewhat 
better results on a weight-for-weight basis. It is quite 
possible that still lighter bombs might be effective, 
but the results would depend on various factors, such 
as the percentage of explosive weight to total weight, 
weight of equipment required to drop longer sticks, 
etc. 

3. A comparison of the curve for the 350-lb Torpex 
flat-nose bomb with the curve for the corresponding 
TNT-filled bomb shows the value of using a more 
powerful explosive. Improvement of lethal radius 
for a given weight of bomb by means of an improved 


explosive is a very effective means of increasing the 
lethality of attack. 

4. The curves show that for the bombs considered 
(and under the assumptions made) the 350-lb flat- 
nose, Torpex-filled depth bomb is superior for any 
given weight of bombs dropped. For the other types, 
the contact bomb of Hedgehog type appears superior 
for weights under about 1000 lb, but this may be 
only an apparent advantage. Depth bombs explod- 
ing outside lethal range may still damage the sub- 
marine, whereas a near miss with a contact bomb 
does no damage. Furthermore, the extra weight re- 
quired by more numerous bomb racks for small con- 
tact bombs will also lessen their advantage. Since 
only Class A attacks have been considered, it may 
seem that the advantage of contact bombs in covering 
all possible submarine depths has been disregarded 
in our analysis. Actually, however, consideration of 
later attacks would add little to the probabilities for 
contact bombs because of the uncertainty of subma- 
rine position. 

The above results are based on fixed assumptions 
as to aiming errors. The effect of such errors is illus- 
trated in Figure 14 for a stick of six 350-lb Torpex 
flat-nose depth bombs. Probability of sinking has 
been averaged over all angles of attack with best 
stick spacing for each angle. Results are shown ac- 
cording to the probable range error for a beam at- 
tack. Line errors for beam attacks and both line and 
range errors for other angles of attack have also been 
varied so as to be proportional to the range error 
indicated, using the relationship reflected in Table 3. 
This curve shows very strikingly the importance of 
accuracy in aiming. For range errors under 50 ft, an 
attack is almost certain to be successful, but the prob- 



Figure 14. Probability of sinking as a function of the 
expected errors in bombing for six 350-lb Torpex flat- 
nose depth bombs. 




136 


ATTACKS BY AIRCRAFT 


Table 4. Effect of degree of submergence at attack, 
(Independent attacks by United States aircraft.) 


Degree of 


July- 

Dec 1942 

Jan-July 1943 

Submergence 

No. 

%A- 

D %AorB 

No. 

%A-D 

7o A or B 

Fully surfaced 

17 

41 

24 

52 

37 

23 

Decks awash 

7 

29 

0 

5 

40 

20 

Stern and/or conning tower 

22 

32 

5 

25 

20 

4 

Periscope 

3 

0 

0 

2 

0 

0 

Down 0-15 sec 

38 

3 

3 

11 

18 

9 

Down 15-30 sec 

25 

4 

4 

11 

0 

0 

Down 30-45 sec 

12 

0 

0 

4 

0 

0 

Down over 45 sec 

22 

0 

0 

5 

0 

0 

Other 

9 

8 

8 

33 

30 

21 

TOTAL 

167 

5 

5 

150 

25 

15 


ability drops off rapidly thereafter. For errors of 
about 125 ft it is down to about 50 per cent. The 
value of an effective bomb sight is clearly indicated 
by these figures. 

A priori probabilities of success with eight solid- 
head rockets, for effective angles of attack, are of 
about the same order of magnitude as corresponding 
probabilities of success with six depth bombs using 
the seaman’s eye method of aiming. For example, 
using aiming errors (standard deviation) of 35 mils 
in azimuth and 25 mils in elevation (which are about 
twice the errors obtained in practice), the probability 
of a kill on a Class A U-boat at 400-yd range in a 20- 
degree dive with eight rockets is about 50 per cent. 
This is about the same as the probability shown in 
Figure 14 for six United States Torpex flat-nose 
depth bombs with a range error of 120 ft. The above 
comparison, of course, involves certain specific as- 
sumptions as to aiming errors and weapon character- 
istics. Future improvement in aiming methods and, 
possibly, in weapon design may affect the two weap- 
ons differently and it is, therefore, not possible to 
conclude which will ultimately be the better. Actu- 
ally, it is not necessary to decide this point since they 
are complementary in nature. Depth bombs are ef- 
fective at all angles of attack and for attacks made 
shortly after submergence, while rockets are restricted 
to angles near the beam and to use on visible sub- 
marines. On the other hand, rockets are light in 
weight and provide an effective weapon for small 
planes which cannot carry bombs. They also provide 
an additional punch for bombing planes, giving 
them something like twice the effective weapon 
capacity with only a small increase in load. 


12 3 OPERATIONAL EXPERIENCE 

Actual results achieved in aircraft attacks during 
World War II verify many of the a priori conclusions 
drawn in the previous section. First to be considered 
is the importance of making prompt attacks and of 
using a depth setting appropriate for such attacks. 

12.3.1 Effect of Degree of Submergence 

at Attack 

Table 4 shows the results achieved by independent 
United States aircraft attacks in the Atlantic and 
Mediterranean during the last half of 1942 and the 
first 7 months of 1943. The percentage of A-D attacks 
(U-boat sunk or damaged) and that of A or B attacks 
(U-boat sunk or probably sunk) are shown for each 
period according to the degree of submergence at 
attack. 

The marked improvement in overall results from 
the first period to the second is clearly associated with 
a much greater proportion of attacks on fully sur- 
faced U-boats. The detailed breakdown for each 
period shows quantitatively the correctness of the 
conclusion reached in our theoretical discussion as to 
the decrease in probability of success with increase in 
degree of submergence. 

12.3.2 Importance of 25-ft Depth Setting 

When the United States entered World War II, 
depth settings of 50 ft were common. By the latter 
half of 1942, the importance of a shallower setting 
had been recognized and the usual depth setting was 




OPERATIONAL EXPERIENCE 


137 


25 ft. However, 39 attacks during July-December 
1942 involved 50-ft settings. These resulted in 3 per 
cent A-D assessments as compared with the overall 
figure of 1 1 per cent shown in the above table. During 
the first half of 1942, when the deeper setting was 
generally used, a total of 174 independent aircraft 
attacks on U-boats in the United States Strategic Area 
resulted in only 4.6 per cent A-D assessments. Un- 
doubtedly the deeper setting was at least partly re- 
sponsible for this poor showing. Operational experi- 
ence thus bears out clearly the importance of making 
prompt attacks with a shallow depth setting. 

Since the 25-ft setting for United States depth 
bombs actually produced explosion at depths greater 
than 25 ft on the average, the improvement noted 
above was only a partial realization of that which 
was theoretically possible. Tests indicated that dur- 
ing the period January-July 1943 the current United 
States depth bomb when set for 25 ft actually fired at 
depths between 27 and 64 ft, with an average of about 
40 ft. On the other hand, the British depth bomb 
fired at very nearly the correct depth. A comparison 
of operational results with the two different bombs 
should therefore indicate the advantage of the one 
which exploded at the desired depth. Such a compari- 
son is made in Table 5. The British figures are based 
on daylight attacks on fully or partly surfaced U- 


Table 5. Operational results with different depth bombs. 
British depth bomb. 


Theoretical results 

Operational results 

- 

No. of 
bombs 

Per cent 
kill 

No. of 
bombs 
(average) 

Per cent 
kill 

Ratio of 
operational 
to 

theoretical 

2 

20.5 

2.5 

20 

98% 

4 

34 

4 

28 

82% 

6 

51 

5.9 

37 

73% 

8 

57 

7.8 

55 

96% 

United States depth bomb. 

Theoretical results 

Operational results 






Ratio of 



No. of 


operational 

No. of 

Per cent 

bombs 

Per cent 

to 

bombs 

kill 

(average) 

kill 

theoretical 

2 

23.5 

2.4 

11 

47% 

4 

41.5 

4 

27 

65% 

6 

54 

5.7 

33 

61% 

8 

66 

8.3 

36 

55% 


boats during the period April-October 1943. The 
United States figures are based on attacks for the 
period January— July 1943, including all degrees of 
submergence. To compensate for the inclusion of 
attacks on submerged submarines. United States at- 
tacks assessed A-D (sunk or seriously damaged) are 
considered successful. (Since 72 per cent of the 
United States attacks were on fully or partly visible 
U-boats, whereas kills were only 54 per cent of the 
total A-D assessments, this actually results in over- 
compensation, and the comparison is therefore some- 
what too favorable to the United States depth bomb.) 

It can be seen from the above that the British bomb 
gave results close to expectation, whereas the United 
States bombs averaged a little over half the expecta- 
tion. 

12.3.3 Importance of Bombing Errors 

Next to be considered are operational bombing 
errors and improvement in them due to practice and 
the use of bombsights. As to the actual size of such 
errors a British analysis of 43 photographed daylight 
attacks made by seaman’s eye on visible submarines 
during the period March-October 1943, showed 
mean errors about the conning tower of 141 ft in 
lange and 71 ft in line. These errors were not uni- 
formly distributed about the conning tower. System- 
atic errors in estimating submarine motion were ap- 
parently negligible but there was a systematic over- 
shoot along the aircraft track resulting in an MPI 
which was 86 ft over in range. The MPI in line was 
reasonably near the conning tower, i.e., 13 feet left. 
The errors showed considerable variation with angle 
of attack; the measured results are shown below. It 
was found that the photographic sample was biased 
since track attacks with small line error rarely gave 
satisfactory photographs because explosion plumes 
obscured the U-boat. Therefore, the line error in 
track attacks shown in Table 6 is somewhat too large. 
Attacks on submarines which had submerged less 
than 15 sec before attack gave average errors of 192 
ft in range and 73 ft in line. The sample, however, 

Table 6. Operational bombing errors (visible U-boats). 


Average Average 

line error (feet) range error (feet) 


Beam 

75 

Quarter 

75 

Track 

66 


138 


ATTACKS BY AIRCRAFT 


was small and results are not very reliable. Compari- 
son of those measured errors with those assumed in 
Table 3 for theoretical calculations shows reasonable 
agreement, with the assumed errors somewhat 
smaller than those actually made in operations. 

The effect of practice in reducing bombing errors 
cannot be evaluated explicitly from operational re- 
sults because of the lack of sufficient photographs. 
The importance of continued practice is, however, 
illustrated by Coastal Command experience for the 
period May-December 1943. Table 7 shows a com- 
parison of results for this period according to amount 
of practice. 


Table 7. Effect of practice on attack accuracy. 



Bomb aimers who had 
dropped less than 10 
practice bombs during 
preceding month 

Bomb aimers who had 
dropped more than 10 
practice bombs during 
preceding month 

Good attacks 

51% 

65% 

Moderate attacks 13% 

20% 

Bad attacks 

36 % 

15% 


The effect of a bombsight on bombing accuracy is 
illustrated by British experience with the Mk III 
(angular velocity) sight. An analysis of results 
through June 1944 showed average range errors 
about the conning tower of 130 ft as compared to 180 
ft for seaman’s eye bombing under the same condi- 
tions. Average line errors were 33 ft as compared to 
56 ft by seaman’s eye, though there is no reason to 
expect an improvement in line error. There was an 
increase in lethality of about 35 per cent in kills 
and 60 per cent in kills and damage. These results 
are based on a very small sample (32 attacks, 16 
photographs) and therefore are not conclusive; how- 
ever they do show the bombsight to be a promising 
development. 

12.3.4 Importance of Large Bomb Stick 

Operational experience has clearly borne out the 


advantage of dropping large sticks of bombs. For 
example, in the period from July 1942 to July 1943 
United States results for depth bombs set at 25 ft 
were as given in Table 8. 


Table 8. Success of attack for different sizes of stick. 
(United States aircraft, July 1942 -July 1943.) 


Number of bombs 

Percentage A-D assessments 

1-3 

13 

4 

21 

5-12 

34 


Results with Rockets 

Operational results with rockets are not very ex- 
tensive. Almost all United States rocket attacks were 
made in connection with other types of attacks so 
that no specific conclusions can be drawn from them. 
British attacks, however, demonstrate that rockets 
have been effective. For example, during the period 
May-December 1943, 18 attacks on Class A U-boats 
(of which 14 were fully surfaced) resulted in 33 per 
cent A or B assessments and an additional 22 per cent 
C, D, and E assessments. The 33 per cent kills is in 
fair agreement with the 50 per cent expected figure 
quoted in our theoretical discussion when considera- 
tion is given to the fact that the average firing range 
was 600 yd instead of 400 yd, the average number of 
projectiles 7.3 instead of 8, and the average glide 
angle 17 degrees to 25 degrees instead of 20 degrees. 
The above figure of 33 per cent kills is comparable to 
the result previously shown for British depth-bomb 
attacks on visible submarines, using six bombs, for 
the period April-October 1943, namely 37 per cent. 
(See Table 5.) It is the same as the figure previously 
shown for six United States depth bombs (based on 
A-D assessments, all attacks). There seems little 
doubt, therefore, that rockets are of about the same 
degree of effectiveness as depth bombs, although, as 
pointed out previously, each weapon has its own par- 
ticular advantages. 




Chapter 13 

OFFENSIVE SEARCH 


P REX’ious CHAPTERS liavc discussed problems in- 
volved in the defense of ships and convoys and in 
attacks on submarines by surface craft and aircraft. 
To some extent these attacks may be made on sub- 
marines contacted during escort of convoy opera- 
tions, but these operations are not the only source of 
contacts. Normally, offensive operations are also un- 
dertaken for the specific purpose of contacting and 
attacking submarines in order to inflict a high loss 
rate on the enemy submarine fleet. While the overall 
aim of all antisubmarine operations is a negative 
one— to prevent the submarines from accomplishing 
their objective— this aim can be achieved by both de- 
fensive and offensive means. Convoy escort is clearly 
defensive, but once convoy escorts make a contact, an 
effort is made to attack and sink the sub, that is, to 
take the offensive. Sinking the submarine is valuable 
in a direct defensive way in that it is then certainly 
prevented from attacking the convoy, but a sinking is 
also valuable offensively in that the submarine is 
thereby eliminated from all future operations. The 
clearly offensive phase of antisubmarine activity 
takes the form of searching for submarines and at- 
tacking them even when they do not immediately 
threaten any friendly ships. The distinction is not 
absolutely hard and fast, since any submarine is a 
potential threat. Defensive measures are intended to 
find and attack submarines that may be dangerous 
within the next few minutes or hours, offensive meas- 
ures, those dangerous at more remote time. 

Methods for attacking submarines are the same 
whether the intent is offensive or defensive, but the 
methods involved in searching for a submarine de- 
pend on the aim of the operation. In particular, de- 
fensive operations are carried out fairly close to the 
ships being defended, whereas offensive operations 
are concentrated in the regions containing the most 
subs, other things being equal. This chapter will out- 
line some of the considerations involved in conduct- 
ing offendve searches. Three general types of situa- 
tion are involved. 

1. Search of an area in which one or more sub- 
marines are thought to be patrolling. 

2. Interception of submarines in transit whose 
paths are thought to pass through a certain region. 


3. Follow-up of contacts made some time previ- 
ously and then lost, for the purpose of finding the 
submarine again. 

These problems have been discussed in general 
terms in Volume 2B, Search and Screening, Chapters 
3, 7, and 8, but not with special reference to searching 
for submarines. In these chapters detailed methods 
are described for designing searches in a great variety 
of tactical situations, and the basic theory of search 
is developed, which applies to search for submarines 
as a special case. Consequently the following discus- 
sion is, in a sense, a review of material presented in 
Search and Screening, with emphasis on the antisub- 
marine applications and operational data reflecting 
experience in antisubmarine warfare [ASW]. 

*3 1 SEARCH OF AN AREA 

When available intelligence indicates* that sub- 
marines are on patrol in a certain region, search for 
them will be productive of contacts in proportion to 
the density of submarines and to the area which can 
be covered by the searching craft. The general con- 
siderations of Chapter 3 of Search and Screening 
apply, assuming the available intelligence to be ex- 
pressed as a probability density function. For a given 
function ps(x,y), the chances of success are deter- 
mined by the search rate and the amount of search- 
ing effort available. In order to have an effective 
search the searching craft must either be many in 
number or have a large sweep rate. 

13.1.1 Aircraft Search for Surfaced 
Submarines 

Aircraft are outstanding for having a large sweep 
rate because of their high speed and distant visual 
horizon.^ They are normally restricted to visual and 
radar detection, however, and are thus effective only 


a While the accuracy of intelligence is of great importance in 
deciding on the plan of any search operation, since it determines 
the area that should be searched, analysis of the accuracy of dif- 
ferent types of intelligence is beyond the scope of this discussion, 
b See Chapters 4 and 5 of Volume 2B, Search and Screening. 


140 


OFFENSIVE SEARCH 


against surfaced (or Schnorcheling) submarines.*^ 
Consequently the density which must be used in 
studies of aircraft search is the density of surfaced 
submarines, and a region in which there are many 
submarines all of which are submerged is not profit- 
able for search by aircraft. 

As an example of the influence of the submarine’s 
submergence tactics, the offshore gain effect can be 
used to illustrate the significance of sweep rates. It 
was common experience that aircraft patrol was 
relatively most effective when carried out at a con- 
siderable distance from shore bases, since submarines 
close to them were cautious and spent a large fraction 
of the time submerged. As an example of the phe- 
nomenon, data based on Moroccan Sea Frontier fly- 
ing are of interest. 

The region under study was divided into zones 
200 miles wide, that is: 0-200 miles from land, 200- 
400 miles, 400-600 miles, and 600-800 miles. The 
number of actual aircraft contacts in each zone was 
counted, and an expected number computed, which 
was based on the number of flying hours in each zone, 
the total density of U-boats in the zone, and an as- 
sumed 5-mile sweep width. In this way the hgures of 
Table 1 were derived. The effective sweep width is 


Table 1. Contacts in Moroccan sea frontier antisubmar- 
ine patrols. 


Distance from base 
(miles) 

0-200 

200-400 

400-600 

600-800 

Total 

Expected No. contacts 

83 

58 

30 

20 

191 

Actual No. contacts 

5 

5 

10 

6 

26 

Per cent realized 

6 

9 

33 

30 

14 

Effective sweep width 
(miles) 

0.3 

0.5 

1.6 

1.5 

0.7 


the value of sweep width that would have to be used 
in computations to have the expected number of con- 
tacts equal to the number actually obtained. The in- 
crease in effective sweep width or “per cent realized’’ 
as distance from shore bases is increased is probably 
due to a relaxation of precautions by the U-boats. 
Whatever its cause, however, it is evidence of the 
desirability of flying at considerable distance from 
shore bases when on offensive patrol. 

The use of radar to increase the sweep width be- 

c Special gear in the form of sonobuoys or magnetic anomaly 
detectors [MAD] are available for detection of submerged sub- 
marines. They will be discussed later, since the sweep rates are 
very small. 


yond that achievable by visual means is of great im- 
portance in area search (or in any other type). Early 
radars did not have a detection range sufficiently 
great to exceed that of visual detection except in 
periods of darkness or low visibility, but the newer 
types are powerful enough to do so a large part of 
the time. The older radar was valuable chiefly be- 
cause it made contacts in periods of low visibility 
when U-boats were surfaced. Data on Army Air 
Forces Anti-Submarine Command flying in Eastern 
Sea Frontier for May through October 1942 and in 
the Trinidad area for October and November both 
bear this out, as is shown in Table 2. In these cases 


Table 2. Radar versus visual search during 1942 (in terms 
of hours per contact). 



Visual 

only 

Radar 
during day 

Radar 

during night 

ESF May-Oct 

Hours of flying 

21,108 

4,665 

1,125 

No. of contacts 

32 

10 

7 

Hours per contact 

660 

466 

161 

Trinidad Oct-Nov 

Hours of flying 

1,400 

2,400 

430 

No. of contacts 

3 

4 

9 

Hours per contact 

470 

600 

50 


use of radar during the daytime did not do very 
much to reduce the number of flying hours required 
to secure a contact, but radar used at night was many 
times more effective than in daytime. The chief ex- 
planation is, no doubt, that U-boats were submerged 
in these regions during the daytime and good oppor- 
tunities for contacting them were offered only at 
night. 

Patrol height is also of importance in achieving the 
maximum sweep rate. For both visual and radar 
search the altitude must be sufficiently high to give a 
distant horizon, and for visual search the increased 
apparent area of the wake is also of importance, as 
explained in Chapter 4 of Volume 2. The theoretical 


Table 3. Sweep widths for aircraft visual search. 
(Surfaced submarine under way.) 


Altitude (feet) 

Visibility 3 miles 

Visibility 15 miles 

500 

3.0 

8 

1,000 

3.5 

10 

2,000 

3.5 

11 

5,000 


12 

10,000 


14 


SEARCH OF AN AREA 


141 


table of sweep rates presented in Table 3 shows 
clearly the advantages of high altitude for visual 
search. 

As an example of operational data confirming the 
value of flying at fair height, data on sightings in the 
Bay of Biscay area during May 1943 can be quoted. 
Table 4 gives the sightings per 100 sorties. As would 
be expected, altitudes over 2000 ft give only a slight 
gain when meteorological visibility is low but have 
considerable advantage under good visibility condi- 
tions. 

The question may well be raised, however, 
whether the high altitude is not a handicap in de- 


livering an attack. The aircraft must lose altitude to 
make a low-level bombing run, which would some- 
times slow up the approach. The overall results 
shown in Table 5 do not, however, indicate any sig- 
nificant effect of this sort. It is probable that U-boat 


Table 4. Sightings per 100 sorties for different patrol 
heights. 



Meteorological visibility 

Height of patrol 

0-4 miles 

5-12 miles over 12 miles 

0-2,000 ft 

6.2 

It 22 

Over 2,000 ft 

7.9 

26 37 


Table 5. Effect of altitude on success of attacks. 


Height 
of patrol 
(feet) 

Percentage of Class A attacks 
(Coastal Command data, June-Nov 1943) 

Per cent of attacks causing damage 
(U. S. Strategic Area July-Dee 1942) 

Total No. 
sighting 

No. of 
Class A 

Per cent 
Class A 

Total No. 
attacks 

No. assessed 
A-D 

Per cent 
A-D 

0-2,000 

96 

44 

46 

91 

10 

11 

2,000-4,000 

86 

34 

40 

36 

3 

8 

Over 4,000 

38 

18 

41 

32 

3 

9 


lookouts have tended to scan the horizon in search 
for aircraft so that the higher-flying aircraft have 
somewhat of an advantage in approaching unde- 
tected. In any event the overall gain of altitude above 
2000 ft is clear. 

There are many other questions relating to the 
most effective conduct of visual or radar search of a 
submarine patrol area by aircraft. The general prob- 
lem involved is to achieve the best possible sweep 
width and then lay out a patrol where surfaced sub- 
marines are densest in accordance with Chapters 3 
and 7 of Volume 2B, Search and Screening. 

The advent of Schnorchel drastically reduced the 
aircraft’s search capabilities because both radar and 
visual sweep width on Schnorchel are much less than 
on surfaced U-boats— according to most estimates 
only 1/10 to 1/100 as great. To a slight extent this 
reduction is compensated by the Schnorcheling U- 
boat’s need to operate in restricted focal areas, the 
area to be searched by either aircraft or surface craft 
being thus much reduced. A net result has been a 
marked increase in the importance of surface craft 
search compared with that by aircraft for submarines 
operating on Schnorchel. 

To some extent however, aircraft can conduct a 
sonar search by use of sono-buoys. If the submarine is 


proceeding at high speed on Schnorchel and is there- 
for noisy, the effective sweep rate of ^ group of sono- 
buoys monitored by an aircraft may be comparable 
to that of surface craft using sonar. 

13.1.2 Surface Craft Search and Submerged 
Submarines 

The role of surface craft in searching an area is 
limited by their small sweep rate. Visual and radar 
detection ranges are shorter than for aircraft, and the 
‘speed of the ship very much less. Table 6 presents a 


Table 6. Sweep rates under various conditions (in sq 
miles per hour). 



Surfaced sub 

Submerged sub 

.Aircraft 

Visual (good visibility) 

1,250 

Approx 0 

Radar— ASG 

2,500 

Approx 0 

MAD 

25* 

15-20* 

Sono-buoys 

200* 

15* 

Surface craft 

Visual 

Approx 0 

Approx 0 

Radar (10 cm) 

100 

Approx 0 

Sonar 

15 

15 


* Estimated on the basis of tests but not confirmed by operational 
data. 


142 


OFFENSIVE SEARCH 


comparison of typical aircraft and surface craft sweep 
rates. Visual search by surface craft is not likely to be 
effective since the submarine can almost invariably 
see the surface craft in time to dive before the surface 
craft sees it. This visual search has purely hold-down 
value and is not effective in an offensive sense. Radar 
search by surface craft under low visibility conditions 
may, on the other hand, be expected to lead to con- 
tacts on any surfaced submarines unless they are 
fitted with search receivers and dive upon being ap- 
proached. For surfaced submarines, however, the 
surface craft have a considerably smaller sweep rate 
than aircraft, so that the chief role of surface craft is 
conducting searches for submerged subs, a task for 
which aircraft are not effective. 

The sweep rates for aircraft using magnetic anom- 
aly detectors [MAD] or sono-buoys in hunting sub- 
merged submarines are about the same as that for 
surface craft using sonar, according to Table 6. The 
comparison is not a completely fair one, however. In 
the first place, the aircraft figures are based on trial 
results and have not been completely substantiated 
by operational data. Operational experience has in- 
dicated that classification of MAD and sono-buoy 
contacts is particularly difficult. In the second place, 
neither of them gives an accurate determination of 
the submarine’s position but only a general indica- 
tion of its presence, and it is difficult to make an 
effective attack on contacts of this type. 

The primary task of surface craft in area search is 
thus sonar search for submerged submarines. Such 
search can be effective only if the submarines are con- 
centrated in a small area, since the surface craft sweep 
rate is so small. To search an area 100 miles square 
with five ships requires about 6 days, whereas a single , 
aircraft can search the same area for surfaced subs in 
4 hours. Accordingly there are relatively few circum- 
stances in which surface craft can be employed profit- 
ably for area search. 

To some extent high-frequency direction-finding 
[DF] extends the search capabilities of surface craft, 
since it enables the ship to take a bearing on a sub- 
marine transmitting on high frequency within about 
20 or 30 miles. If the submarines transmit frequently, 
as German U-boats did during the height of the 
Battle of the Atlantic, DF can increase the radar 
sweep rate by a factor of up to 3 or 4 for surfaced subs. 
A DF contact is not quite so valuable as a radar con- 
tact, however, because it is less definitely localized 
and more difficult to convert into an attack. 


fundamentally, then, surface craft search of an 
area is a matter of sonar search. World War II has 
not offered very many opportunities for such search, 
surface craft having been put to effective use mainly 
in follow-up of contacts and in defensive operations, 
and consequently little in the way of operational 
data is available. 

One tactical principle should be emphasized, how- 
ever, that of searching in groups in line abreast. The 
theoretical reason for doing this is that a submarine 
has a good chance of evading a single ship by steering 
to one side at high speed when the approach of the 
ship is detected. Figure 1 shows the effect of such eva- 




Figure 1, Submarine evasion of sonar search. 

sion. The “entrapment triangle” is drawn tangent to 
the sonar detection circle with limiting escape lines 
drawn at angle = sin-^ sub speed/ship speed. In this 
way it is analogous to the submerged approach zone 
except that the submarine is trying to get out rather 
than in. As can be seen from Figure 1 searching in 
line abreast makes such evasion impossible except 
from positions near the ends of the line. Some theo- 
retical considerations concerning search in line 
abreast are given in Chapter 6, Volume 2B. 

There are also many practical reasons for search- 
ing in line abreast. The ships are close enough for 
convenient communications and know the positions 
of their fellow ships at all times. Similarly the ships 
are readily recognizable by aircraft or other forces in 
the afA, even at night. In addition ships are in a posi- 


INTERCEPTION OF TRANSITS 


143 


tion to coordinate during attack once a contact is 
made. 

It is, in fact, almost always valuable to carry out 
offensive operations in groups. If a group of units is 
sent out at onc> on parallel sweeps (or to patrol the 
same area according to some fixed plan), any unit 
making contact has immediate assistance available 
(provided communications are satisfactory), and all 
the other units can, if desired, enter into the process 
of attacking or following up the contact. In this way 
the chances of sinking the submarine, once contact is 
made, are very materially increased over what they 
would be with a single attacking unit. As a balance to 
this gain is the necessary loss in probability of making 
contact which results from concentrating too much 
of the searching effort into a particular area or period 
of time to the exclusion of some others. These two 
considerations must be weighed and the best com- 
promise achieved. When the area to be searched is 
rather large, however, as is usually the case, a search 
by a group of units covers it only very incompletely 
and there is no difficulty with “over-searching.” In 
such a case offensive patrol in groups spaced as closely 
as possible without overlapping of the areas searched 
by individual units is clearly desirable. Groups may 
involve aircraft or surface craft, or both. The basic 
idea is simply to provide for coordinated attack upon 
any contact that is made without appreciable loss in 
efficiency of distribution of search effort. 

13.2 INTERCEPTION OF TRANSITS 

In many cases it is profitable to endeavor to inter- 
cept submarines en route to their patrol area and at- 
tack them before they are able to become dangerous. 
Submarines in transit may be thought of as making 
up a moving density distribution (as opposed to a 
stationary one for those on patrol). This comparison 
is discussed in detail in Chapter 7 of Volume 2B and 
the appropriate modifications in search plans are de- 
scribed. As a result of this movement, the crossover 
type of barrier patrol is usually the most efficient, as 
was pointed out in Volume 2B. 

This movement does not, in itself, make detection 
of the submarines involved any easier, but often re- 
sults in a more accurate estimate of the submarine’s 
position than is possible with a submarine on patrol. 
This may be very strikingly true if the submarines 
are constrained to pass through a relatively i^rrow 
region while on passage to the patrol area. In such a 



case the submarine density is much higher in the 
transit area than the patrol area and consequently 
the former is the better area for exploitation by an 
antisubmarine offensive, other things being equal. 

13.2.1 Yhe Bay of Biscay Offensive 

The outstanding transit area of this type has been 
the Bay of Biscay off the U-boats’ French ports of 
Brest, Lorient, St. Nazaire, and La Pallice. All U- 
boats entering or leaving these ports had to funnel 
through the Bay within reach of British-based air 
cover, so that excellent opportunities for attacking 
U-boats were presented. (See Figure 2.) 

An analysis of the possibilities of an offensive in 
the Bay was made late in 1942,^ some of whose salient 
arguments are discussed here. The basic idea was 
that the U-boats had to spend a fair amount of time 
on the surface crossing the Bay and that therefore a 
“balanced” force capable of carrying out effective 
search both day and night would be able to intercept 

d Operations Research Section— Coastal Command, Report 
No. 204, Air Offensive Against U-Boats in Transit, December 
10, 1942. 




144 


OFFENSIVE SEARCH 


a large fraction of the U-boats and inflict very serious 
losses. (Schnorchel was not to appear until long after 
this time.) By adopting maximum submergence tac- 
tics, the U-boat could reduce its time on the surface 
during any given day very greatly, but a greater num- 
ber of days was required to cross the Bay because of 
the slower speed involved, and the entire passage 
could not be made submerged. The net gain by sub- 
merging is shown in Table 7, assuming a 300-mile 
band covered by aircraft across which the U-boats 
must pass. Thus the U-boat must spend about 13 hr 
on the surface in crossing the Bay even when using 
maximum submergence. 

Table 7. Surfaced days per transit. 


Fraction of Speed 
time spent on 
on surface surface 
(per cent) (kt) 

Speed 

submerged 

(kt) 

Average 

speed 

(kt) 

Total 

time 

(hours) 

Time 

on 

surface 

(hours) 

100 

10 


10 

30 

30 

50 

10 

4.5 

7.2 

42 

21 

20 

10 

2.8 

4.2 

71 

14 

10 

10 

1.5 

2.4 

125 

13 

100 

17 


17 

rs 

18 


If the total number of transits per month is desig- 
nated by T, then the average number of surfaced 
U-boats in the area is (there being approximately 720 
hr per month): 


AT 13 

iVo = ^T. 


The expected number of contacts is 


( 1 ) 


N^Qh^ ( 2 ) 

where Q = aircraft sweep rate, 

h = number of flying hours per month, 

A = area involved. 


In order to contact every transit U-boat once, for 
example, it would be necessary to have N = t, which 
condition can be used with equations (1) and (2) to 
determine the number of flying hours required. 


h 


_ QJi 13 
A 720’'’ 

720 A -.A 


( 3 ) 


The area involved is about 300 miles square so that 
the hours required are as given by Table 8. 


Table 8. Flying requirements to sight each transit. 


Type detection 

Sweep rate () 

Flying hours 
per month 

gear 

(sq miles per hour) 

required 

Meter-wave 

800 

6,000 

Centimeter 

2,800 

1,800 


These figures for flying hours per month can 
readily be interpreted in terms of the number of 
planes required. A long-range plane is normally ca- 
pable of flying about 50 hr per month, so that a force 
of about 40 such planes equipped with centimeter 
radar would be adequate to contact each U-boat 
crossing the Bay. (The flying required to get back and 
forth from base to the active Bay area would not in- 
crease this figure greatly.) 

The comparison of actual results obtained in the 
operations with these predictions is of particular in- 
terest. Since the situation changed rapidly from time 
to time the data presented in Table 9 are given by 
months and divided into three periods correspond- 
ing to various stages of the battle. Average values are 
given for each period. At the end of the third period 
the number of transits dropped off sharply and U- 
boat activity did not again reach the previous level. 


Table 9. Results of Bay of Biscay offensive. 


Month 

Flying 
hours 
on patrol 

U-boat 

transits 

Sightings 
of U-boats 

Per cent 
sighted 

First period 

June 1942 

2,600 

50 

26 

52 

J«h 

3,750 

65 

20 

31 

Aug 

3,200 

80 

37 

46 

Sept 

4,100 

90 

39 

43 

Average 

3.400 

71 

30 

43 

Second period 

Oct 1942 

4,100 

95 

18 

19 

Nov 

4,600 

140 

19 

14 

Dec 

3,400 

130 

14 

11 

Jan 1943 

3,130 

105 

10 

10 

Average 

3,800 

117 

15 

13 

Third period 

Feb 1943 

4,400 

100 

32 

32 

Mar 

4,600 

135 

42 

31 

Apr 

4,200 

115 

52 

45 

May 

5,350 

120 

98 

81 

June 

5,900 

57 

60 

105 

July 

8,700 

78 

81 

104 

Average 

5,500 

101 

61 

60 


INTERCEPTION OF TRANSITS 


145 


The first period involves night flying by Welling- 
tons equipped with Leigh Lights and meter-wave 
radar. Only about 10 aircraft were so equipped, but 
the overall result was to sight almost half the transits. 
Since the number of flying hours per month was only 
slightly more than half that quoted in Table 8 for 
meter-wave radar, this result is not far removed from 
that predicted. Actually the predicted average num- 
ber of sightings would be 40, the operational results 
30— a pretty close agreement. 

During the following months, however, the frac- 
tion of transits sighted declined markedly, reaching a 
low of 10 per cent. To some extent this may have 
been a seasonal effect because of the difficulties of 
flying in winter, but the chief reason for the drop 
was undoubtedly the introduction of search receivers 
on U-boats which could detect meter-wave radar. 
This development permitted the U-boats to surface 
at night with fair safety, and the aircraft available no 
longer constituted a balanced force. (See Chapter 14 
for further discussion.) 

During the spring of 1943 the Leigh Light Wel- 
lingtons were equipped with centimeter radar and 
results during the third period were correspondingly 
improved. By June and July the fraction of transits 
sighted had risen to 100 per cent, a tenfold increase, 
though the number of flying hours in those months 
was only about twice what it had been in previous 
periods. If all the planes had been fitted with centi- 
meter radar, an even higher sighting rate would have 
been expected on the basis of Table 8, since there was 
considerably more flying than the 1800 hours per 
month which should produce 100 per cent sightings. 
Such was not the case, however, and the average 
sweep rate of all planes involved was probably more 
nearly that of meter radar. On this basis 6000 hr per 
month would be required to produce 100 per cent 
sightings, which is in good agreement with the ob- 
served results. The average amount of flying was 
5500 hours per month, which sighted 60 per cent of 
the transits, reaching a peak of 100 per cent during 
the best summer months. 

It may be concluded, then, that the best periods of 
the Bay operations provided returns quite in accord- 
ance with predictions, but that the U-boats were 
quick to find and exploit any weak points in the 
offensive. The highly profitable periods were not of 
long duration. Even during the low points, however, 
Biscay operations were quite profitable compared 
with aircraft patrol in other regions, since the flying 


hours required to make a sighting rarely rose above 
500, whereas in many areas thousands of hours were 
required. 

13.2.2 Value of Interception of Transit 
Submarines 

The final evaluation of any offensive against tran- 
sit U-boats must be made on the basis of a compari- 
son with other possible uses of the forces involved. 
When the other use contemplated is also some type 
of offensive patrol, the chief criterion is that of sub- 
marine density (or density of surfaced subs if more 
appropriate), as is demonstrated in Volume 2B, Chap- 
ter 3. Offensive patrol should be carried out in the 
region of greatest submarine density, subject to cer- 
tain practical considerations. A region in which 
weather conditions reduce the effectiveness of detec- 
tion gear by a factor of three, for instance, would not 
be a profitable one for an offensive unless the sub- 
marine density there was at least three times that in 
other regions. 

When the comparison must be made between of- 
fensive and defensive operations, many more factors 
must be considered. The usual objective measure of 
the effectiveness of antisubmarine operations is the 
number of ships saved. Defensive operations effect 
such a saving quite directly, whereas offensive opera- 
tions have an indirect effect through reduction in 
numbers of submarines operating and through low- 
ering of the morale and state of training of the sub- 
marine crews. The latter effect is very hard to assess 
in any numerical terms. U-boats have usually with- 
drawn from areas in which their chance of being 
sunk was greater than about 10 percent per month 
of those at sea when there was a safer alternative, but 
the high sinking rate has often been associated with 
a low rate of sinking ships, so that the U-boats may 
have withdrawn in search of more profitable areas, 
rather than because of any effect on morale. Never- 
theless, the U-boats’ heavy losses in 1943 were fol- 
lowed by a period of very unaggressive operations, 
even though the losses suffered were not enough seri- 
ously to diminish the size of the U-boat fleet, and 
there is no doubt that a lowering of morale and ex- 
perience of crews had something to do with it. 

In order to illustrate the type of comparison that 
must be made, consider a hypothetical situation in 
which the enemy has a hundred submarines which 
can spend half their time at sea. He can build five 


146 


OFFENSIVE SEARCH 


per month, but we sink the same number by surface 
craft {i.e., 10 per cent of those at sea each month), so 
that a sort of equilibrium has been reached. Given, 
then, a force composed of 40 aircraft, should they be 
employed either offensively, assuming that each sub- 
marine transit would then be sighted in accordance 
with equations (1) to (3), or defensively for escort of 
threatened convoys, assuming that the protection 
offered would be in accordance with the data of 
Chapter 10? 

In each case the number of ships saved by the air- 
craft must be determined. For the offensive opera- 
tions the saving can be estimated in the following 
way: 

The immediate effect of the offensive operation is 
an increased number of submarines sunk per month. 
This will reduce the number of submarines operat- 
ing until, in the long run, a new equilibrium is 
reached. The number of submarines sunk per month 
will then be equal to the number built. If N is the 
number of submarines available to the enemy, and 
T is the length of a submarine patrol in months, 
then: 


N = number of submarines at sea, and 
N 

— = number of transits made per month. 


The capabilities of the surface craft ^re assumed to 
be such that they sink 10 per cent of the subs at sea 
per month, and data from Chapter 12 show that it is 
reasonable to assume that about 10 per cent of all air- 
craft sightings lead to attacks which sink the sub- 
marine. Combining these figures with the ability of 
the aircraft to intercept each transit, we have, at 
equilibrium: 


0.10 X Y + 


OAON 

T 


No. of subs built per mo 


N = 


lOOT 
2 + T 


5; 

( 4 ) 


The normal length cruise for a U-boat has been 
about 2 months. Using this value for T we have ^ 
N = 50 submarines, which is only half the force that 
was available in the absence of the aircraft offensive. 
Thus we would expect this offensive use of aircraft to 
cut the losses of merchant ships in half. 

This figure can be compared with the direct de- 
fensive value in escort of threatened convoys. The 
experience quoted in Chapter 10 was that four sorties 
per day decreased the daily losses by about 65 per 


cent. If the aircraft force available could provide this 
extent of convoy coverage, it would be considerably 
more valuable than when employed offensively, be- 
cause the 65 per cent figure measures only direct de- 
fensive value and does not take into account any 
attacks on submarines that the planes might make 
while on convoy escort duty. The total amount of air- 
craft flying required for the convoy escort would not 
generally be excessive. During the actual period 
studied previously there were about 20 convoy-days 
per month in the “threatened” class, so that only 80 
sorties per month would be needed, whereas the air- 
craft could be expected to fly about 200 sorties 
monthly. Some threatened convoys may, however, be 
in positions which cannot readily be covered by air- 
craft, so that the expected thoroughness of coverage 
must be estimated on the basis of weather, distance of 
convoy routes from bases, and similar factors. In the 
North Atlantic, convoys were, in fact, covered on 
somewhat less than half the days when they were 
threatened. If this sort of restriction must be ac- 
cepted, the expected reduction in losses is only about 
30 per cent, which is no longer obviously superior to 
the offensive employment of aircraft in the transit 
area. As a matter of fact the sightings made (about 
15 per month) were about enough to make up the 
discrepancy between 30 per cent and 50 per cent 
saving. 

Such a comparison is never strictly valid, however, 
because it is made on the basis of an equilibrium con- 
dition which is not approached very quickly. If we 
consider N to be a function of time, then equation (4) 
is replaced by: 


^ = 0.10 X 1 + 0.10^ -5 (5) 


= (0.05 + 5^)Af - 5 


N ^ ^ ^ -f 


0.05 -f 


0.10 


( 6 ) 


= Ne 


-( 


0.05 


0 . 10 \ 
T ) 


+ ^eq- 


Here is the equilibrium number of submarines. 
For the case previously considered, T = 2 months, 
the number of submarines is plotted in Figure 3. 
Thus the value of an offensive operation depends 


FOLLOW-UP OF CONTACTS 


147 



Figure 3. Diminution of submarine fleet by an anti- 
submarine offensive. 


Oil the expected length of the war, the upper curve 
giving the reduction in the average number of subma- 
rines operating as a function of the length of period 
under consideration. If the war is expected to last 
another 30 or more months, the offensive will achieve 
very nearly its equilibrium effectiveness in that the 
number of submarines operating is reduced by about 
40 per cent with the lifetime (and consequently ex- 
perience of ships and crews are correspondingly 
reduced). If, on the other hand, the war is to last only 
6 months, the average reduction in submarines oper- 
ating is only about 10 per cent. 

The comparison between defensive and offensive 
measures is made explicitly in Figure 4. The ordinate 
is proportional to the reduction in ship losses, with 
no account taken of the attacks on U-boats made by 
escort aircraft. It is clear that the effect of the length 
of the war on the value of an antisubmarine offensive 
and the effect of the probable completeness of convoy 



A 








B 








IN TRANSIT 01 

FFENSIVE 



0 10 20 30 
LENGTH OF WAR IN MONTHS 


Figure 4. Relative saving of ships for various uses of air- 
craft. (A) Defensive value of escort of all threatened con- 
voys; (B) defensive value of partial escort of threatened 
convoys. 


escort must be taken into account in arriving at an 
evaluation of the relative merits of the two possible 
uses of the aircraft. It is clear also that no general 
conclusion can be made, but that each decision must 
be made after a consideration of the tactical and 
strategical situation obtaining at that time. 


FOLLOW-UP OF CONTACTS 


Probably the most important aspect of offensive 
operations is the attempt to convert the largest pos- 
sible fraction of all contacts into attacks and kills. 
Often the manner in which contact is made is such 
that an immediate attack is not possible. (A typical 
example would be a sighting by a merchant ship or 
nonoperational aircraft.) It may be that an attack has 
been made on the sub and contact subsequently lost, 
so that contact must be regained in order to con- 
tinue the attack. In these cases it is usually very 
profitable to conduct a follow-up search in order to 
find the sub again. 

Follow-up of contacts is a profitable employment 
of antisubmarine forces because for the first few 
hours after contact has been lost the region in which 
the submarine can be is rather small. In this region 
the probability density is accordingly high and it 
therefore constitutes a profitable region for search. As 
an example, consider the case of a submarine whose 
position is definitely known at time t = 0, and is then 
lost. The submarine is free to travel on the surface. 
Since its speed is certainly not greater than 20 knots, 
we can be sure that it is within a circle of radius 20^ 
miles. The corresponding submarine probability 
density is shown in Figure 5, calculated on the as- 
sumption that the submarine is equally likely to be 
anywhere within the circle, that is, that 


^ x(20<)^’ 


( 7 ) 



TIME IN HOURS 


Figure 5. Local probability density after a submarine 
contact; calculated from equation (7). 


148 


OFFENSIVE SEARCH 


1 he horizontal line drawn for comparison shows a 
typical average U-boat density for an active area dur- 
ing the Atlantic war. The conclusion is that follow-up 
of the contact is more profitable than general area 
patrol for about 10-20 hours after the contact, in this 
case. If the overall sid^marine density is increased, 
follow-up of contact becomes relatively less profit- 
able, whereas in a region of low density it is of utmost 
importance to exploit each contact to the full. 

Figure 5, drawn for submarine speeds of 10-20 
knots, is applicable for surfaced submarines. If the 
submarine is held down by aircraft or surface craft, its 
speed may be considerably less, normally only 2-3 
knots for conventional types of submarine. In this 
case the local density remains high for a much longer 
period than is shown in Figure 5. Consequently, 
sonar search for submerged submarines is very much 
more effective in follow-up of previous contacts than 
in routine area search. Only in areas of great sub- 
marine density is it profitable to employ surface craft 
for area search by sonar. In general the chief offensive 
employment of surface craft will be to follow up 
previous contacts, appropriate plans and tactics for 
doing so being of prime importance. 


There are, then, two distinct tactical situations, 
according as aircraft or surface craft are the principal 
agents of search. If the hunt is being carried out by 
aircraft alone, the aim is to recontact promptly any 
submarine which dares to show itself on the surface. 
Wffien surface craft are present, however, the sid^ma- 
rine will normally remain submerged. (A coordi- 
nated hunt by aircraft should be carried out when- 
ever possible to ensure that the submarine does not 
make a high-speed escape on the surface.) So long as 
the submarine’s submerged speed is low, the surface 
craft’s sonar search has a good chance of resulting in 
recontact. Of the two problems, that of surface craft 
search is somewhat the simpler and will be discussed 
first. 

In the actual design of such follow-up searches for 
surface craft, two primary requirements must be met 
In the first place, the search must be as simple as 
possible to carry out and involve no more turns or 
changes in disposition than necessary. Ships should 
be kept in a line-abreast formation unless there is 
some special reason for doing otherwise. Standard- 
ized turns and maneuvers should be employed to 
avoid confusion in operation. In addition the plan 



FOLLOW-UP OF CONTACTS 


149 




PATH OF 
SHIPS 


Figure 7. Typical plan for sonar search by surface craft. 


flat-topped cylinder, that is, to assume that the sub- 
marine is equally likely to be anywhere within a cer- 
tain radius of the initial point and is known to be 
somewhere within that radius. It is reasonable to 
write the radius as 

r = 

where a = uncertainty in initial position, 

V = submarine’s speed, 
t = time since initial contact. 

Assume that the increment of probability of having 
made contact can be written 

“ n(a^+v^l^) 

This assumption implies that the search is carried 
out at random, but can be used as a fair approxima- 
tion to an actual search plan. (See Volume 2B, Chap- 
ter 3.) Then 


must search regions in which the submarine is likely 
to be, in general accordance with the rules of Volume 
2B, Chapter 3. In designing the plan, then, an esti- 
mate of probability density distribution as a function 
of time must be made. Immediately after the original 
contact the density is very highly peaked, the only 
dispersion arising from errors in reporting the con- 
tact position, but as time goes on the submarine may 
move further and further from the position of the 
original contact and will probably be at some dis- 
tance from it. A typical set of densities is shown in 
Figure 6, and further discussion of such distributions 
may be found in Volume 2B, Chapter 1. 

It is evident that a search plan drawn up on the 
basis of such an expanding distribution will usually 
have the general shape shown in Figure 7. Extensive 
sets of such plans are to be found in present doctrine.® 

It is not necessary to go through a detailed analysis 
of probability functions such as those of Figure 6, 
however, to gain a general idea of the effectiveness of 
surface craft follow-up tactics. An analysis of the fol- 
lowing type is adequate to indicate the importance of 
the factors considered. The fundamental simplifica- 
tion is to replace the surface shown in Figure 6 by a 

e Plans are presented in FTP 223A. The mathematical basis 
upon which such plans are constructed is given in Volume 2B. 
Chapters 3 and 7. 


P = 


1 — const X e 


— (Q/tTow) tan"^(t)f/o) 


( 8 ) 


The constant of integration in equation (8) is deter- 
mined by the condition thatp = 0 at the time of com- 
mencing search. If this time is denoted by 

, ■. — (Q/TTar) [ tan"^ (I't /o) — tan“^ (rt, /o) ] 

p = \ - e " . 


When a search is being conducted it should normally 
be carried out until there is nothing to be gained by 
further searching, i.e., until p{t) is very near p{^), 
which is the ultimate limit. Hence it is useful to write 
the probability for ^ = oo, which is 


p(oo) = 1 — 


-(O/TTaa) [(7r/2)_tan-l(«ti/a)] 


( 10 ) 


Some typical curves plotted on the basis of equa- 
tion (10) are plotted in Figure 8. A sweep rate of 30 
square miles per hour and a sub speed of 3 knots are 
assumed. It is evident that a prompt start on the 
search combined with accurate localization of the 
contact are required for a good probability of success. 

Operational data are available on success of follow- 
up hunts aimed at regaining contact with a subma- 
rine after lost contact following an attack; these can 
be compared with the theoretical predictions. For 


,--y^^TTn snm7ir7?-( 


150 


OFFENSIVE SEARCH 



Figure 8. Probability of catch for: sweep rate 30 sq miles 
per hour; sub speed of 3 kt. 


cases in which a submarine was believed to be pres- 
ent, the results are presented in Table 10. An aver- 
age of about two ships took part in the searches. 


Table 10. Success of searches to recontact U-boat. 



Average time 



from lost contact 

Per cent 

No. of cases 

to start of search 

successful 

19 

22 min 

53 

12 

90 min 

25 


These figures are plotted in Figure 9, and a calcu- 
lated curve is drawn which is in fair agreement with 
the operational points. This curve is drawn on the 
assumption that sub speed is 3 knots, initial uncer- 
tainty is 1 mile, and sweep rate is 10 sq miles per 
hour. The first two assumptions are reasonable for 
the tactical situation involved. Since the problem is 
one of recontacting a submarine which has been pre- 
viously attacked, a location error of more than 1 mile 
is unlikely. The sweep rate of only about 5 sq miles 
per hour per ship is somewhat smaller than might be 
expected, however. In Volume 2B, Chapter 6, the 
average sonar sweep width is estimated at 1800 yd, 
corresponding to a sweep rate of about 10 to 12 sq 
miles per hour. Several explanations are possible: (1) 
in this case U-boats may have been especially careful 
to make good use of layer effect and other opportuni- 
ties to escape detection, (2) wakes, depth charge ex- 
plosions, etc., may have made search more difficult, 
or (3) the previous estimate, based on the assumption 
that a submarine would have been detected every 
time at very short range, may have been unduly op- 
timistic. There is, however, no irreconcilable differ- 
ence between the theoretical expectations and opera- 
tional results. 


75 


50 


UJ 

o 

q: 

tu 

a. 

25 


0 

0 0.5 1.0 1.5 ZJO 

TIME IN HOURS UNTIL START OF SEARCH 

Figure 9. Per cent of submarines contacted by surface 
craft hunts. (Circles are operational data on regaining 
contact. (Solid curve is calculated for: Q = 10 sq miles per 
hour, A = 1 mile, F = 3 kt, by equation (10). 



Follow-up hunts by aircraft are of two general 
types. The first of these is aimed at detection of sub- 
merged submarines by MAD or sono-buoys, the sec- 
ond at visual or radar detection of the submarine 
when it has resurfaced. Aside from special limitations 
on maneuverability, those of the hrst type have much 
in common with surface craft hunts, since the subma- 
rine’s behavior is approximately the same. In order 
to have an acceptable chance of success the search 
must be begun very soon after the initial lost contact, 
and the accuracy of locating the initial poin^must be 
high. Since this type of follow-up presents no essen- 
tially new problems, detailed discussion will not be 
given here. 

When, however, the aircraft relies on visual or 
radar detection, the search must be planned with 
recognition of the fact that a submerged submarine 
cannot be detected. Two alternatives are possible, 
either a hold-down hunt aimed at covering all pos- 
sible positions of the submarine so as to keep it from 
surfacing until battery and/or crew endurance are 
exhausted, or a “gambit” procedure in which the 
aircraft endeavor to permit the submarine to resur- 
face and then recontact it. This latter alternative is 
equivalent to designing the search so as to concen- 
trate on those regions in which surfaced submarines 
are most likely to be. In practice the gambit proce- 
dure has usually involved a box search centered at 


FOLLOW-UP OF CONTACTS 


151 



Figure 10. Comparison of follow-up hunts. 


the point of last contact and far enough distant that 
the sub will surface and then try to cross the aircraft’s 
patrol. The hold-down hunt, on the other hand, 
must cover the whole possible area of the submarine 
at frequent intervals. This requires a much larger 
expenditure of effort, actually far more than is usu- 
ally available, as can be seen in Figure 10. 


Hold -down 

1. Aircraft must cover area 
Tr{ut)- about 3 times per hour 
so as to keep sub down at all 
times. 

2. For velocity of aircraft = V , 
and sweep width lo, this re- 
quires 2>Tr (uVf/wV aircraft in 
the region at time = t. 


Gambit 

1. Aircraft must make circuit 

2, Trut in length often enough 
to prevent crossing on surface. 

2. For aircraft velocity = V, 
sweep width w, and sub sur- 
faced speed Ug this requires 
(UJw) {Ziriit/V) aircraft flying 
at time = t. 

= 3 kt, u; = 10 miles, 


3. To carry out from 1 hr to 36 hr with u 
V = 125 kt, and Ug = 15 kt we require 

1050 flying hours 


150 flying hours 


Obviously much less flying is required for the gam- 
bit procedure than for hold-down, especially since a 
true hold-down should last for well over 36 hr. As 
a matter of fact, there have been very few cases where 


Table 11. Hours of flying on contact by type of mission. 
(Trinidad area). 


hunts have been made with sufficient intensity to 
qualify as true hold-downs offering the submarine 720 
opportunity for surfacing. A hard and fast distinc- 
tion cannot be made in the case of actual operations. 

Operational results normally confirm the theo- 
retical expectation that flying on hunts in follow-up 
of contact is more profitable than any other type in 
terms of flying hours required per submarine contact. 
Table 1 1 presents data of this sort from the Trinidad 
Area for the period August 1942 to January 1943. 
Hunt flying is seen to be three to four times as profit- 
able as other types. 

A small number of aircraft hunts have been 
studied in detail in order to determine the relative 
effectiveness of different types of follow-up tactics. 
Although the number of cases involved was too small 
to permit great reliance to be placed on the results, 
they are of interest as confirmation of the expected 
trends. A total of 18 hunts are involved, which oc- 
curred between March 15, 1943, and October 20, 
1943, in the United States Strategic Area. The overall 
results are summarized in Table 12. 


Table 12. Results of aircraft hunts. 
(United States strategic area, 1943.) 


Total number of hunts 18 

Hunts achieving recontacts 1 1 

Per cent of hunts successful 61 

Average duration of hunt (hours) 59 

Total number of recontacts made 22 

Recontacts per hunt 1 ,22 

Average time between recontacts (hours) 14.6 


The effectiveness of these hunts is shown by the 
fact that 61 per cent of them succeeded in again estab- 
lishing contact with the submarine. On the average, 
subs were recontacted about two times in those cases. 
A more detailed breakdown of the results is pre- 
sented in Table 13. 


Total hours of flying Avg hours per contact 



Escort 

of 

convoy 

Routine 

patrol 

Hunts 

Escort 

of 

convoy 

Routine 

patrol 

Hunts 

Aug 1942 

714 

1,514 

134 

714 

116 

45 

Sept 

1,085 

3,405 

486 

362 

227 

97 

Oct 

489 

3,852 

701 

163 

241 

88 

Nov 

925 

3,221 

584 


403 

584 

Dec 

638 

3,171 

620 

319 

3,171 

124 

Jan 1943 

1,038 

3,668 

486 

519 

1,834 

234 

Total 

4,889 

18,831 

3,011 

444 

342 

125 


Table 13. Comparison of results by hunt type. 



Continuous 

gambit 

Modified 

exhaustion 

Exhaustion 

Number of hunts 

12 

2 

4 

Number successful 

9 

1 

1 

Per cent successful 

75 

50 

25 

Average flying hours 

in hunt area 

75 

280 

300 

Number of recontacts 

per hunt 

1.33 

2.50 

0.25 

Flying hours in area 

per recontact 

58 

114 

1,186 


152 


OFFENSIVE SEARCH 


This table shows clearly the high effectiveness of 
the gambit type of hunt in comparison with other 
types. It ranks well in terms of percentage success an- 
average number of recontacts per hunt. Since the 
number of flying hours employed was much smaller 
than for the other types, the results in terms of flying 
hours required per recontact show a definite advan- 
tage to the gambit type. 

In connection with the general importance of fol- 
low-up hunts, one further remark is worth while. 
Such hunts are profitable because they are carried 


out in a small area in which the submarine proba- 
bility density is high. For best effectiveness accurate 
navigation is required to locate the area properly and 
to carry out the desired search plan. In addition com- 
munications must be excellent in order to assure that 
the hunting craft reach the submarine position at 
the earliest possible moment, since the early stages 
of the hunt are the most profitable. In order to insure 
that coordinated follow-up hunts are carried out 
successfully, great skill is required in each of these 
fields. 



Chapter 14 

EMPLOYMENT OF SEARCH RADAR IN RELATION TO 
ENEMY COUNTERMEASURES 


>41 INTRODUCTION 

R adar has been only one of the many weapons 
applied to defeat the German use of U-boats, but 
it has played an important role at certain critical 
times. Because the U-boat high command failed to 
anticipate the effectiveness of search radar, its use by 
the Allies caused especially grave concern to the U- 
boats. The moves and countermoves of the radar war 
offer an interesting example of the importance of 
quick and accurate evaluation of enemy measures. 
Although the events of World War II conform in 
general to the scheme of measures and countermeas- 
ure set forth in Volume 1 on Operations Research, a 
review of them shows that evaluation of the opera- 
tional effectiveness of enemy countermeasures is of 
utmost importance. Even when a countermeasure has 
excellent theoretical performance, it is a rare event 
for it to be applied widely enough and with suffi- 
cient operational effectiveness to justify the extreme 
tactic of abandoning the weapon being countered. 
Usually the prompt (but not premature) application 
of counter-countermeasures will largely restore the 
effectiveness of the weapon. This has been particu- 
larly true of the radar versus search receiver compe- 
tition which arose during the latter part of the U- 
boat war. 

The following discussion will accordingly pay pri- 
mary attention to the development of German search 
receivers and the counter-countermeasures employed 
to reduce the effectiveness of the German gear. In 
addition, the importance of Schnorchel and radar 
camouflage as a countermeasure to Allied search 
radar will be considered. There have also, however, 
been a number of less important German radar 
countermeasure developments, which are described 
briefly below. 

1. Submergence. It is evident that the submarine 
can escape radar detection by staying submerged, and 
in the final phases of World War II German U-boats 
made a policy of keeping below the surface as much 
as possible when in dangerous waters. As pointed out 
in previous chapters, however, the loss in mobility 
and in offensive capabilities was very serious, and a 


normal type of U-boat (without Schnorchel) could 
not operate effectively using maximum submergence 
tactics. Such tactics as these must be considered a very 
unsatisfactory last resort from the submarine’s point 
of view." 

2. Aircraft warning radar. Radar of this type 
(Hohentwiel) was developed and installed on a con- 
siderable number of U-boats. Its use was slight, how- 
ever, and successes insignificant. This was due to two 
factors, the relatively low power and short range of 
this radar and the fear of Allied search receivers. In 
general the range on aircraft of the long-wave, low- 
power U-boat radar was shorter than the range on 
surfaced U-boats of Allied microwave air-to-surface- 
vessel [ASV] radar. This increased the lack of con- 
fidence in radar current among U-boat commanders 
which was caused by their basic objection to any ra- 
diation of energy which could be listened to and 
located by direction-finding devices. They were con- 
vinced of the danger of Allied listening gear long be- 
fore it existed in adequate quantity to be of opera- 
tional importance. This threat was sufficient to nul- 
lify the use of U-boat radar. This fact has been sub- 
sequently established from statements made by Ger- 
man personnel but was appreciated earlier on the 
basis of information obtained from ferret planes 
equipped with listening gear. They were dispatched 
to areas in which U-boats were known to be operat- 
ing, but the sequence of negative reports showed that 
there was no significant use of the radar sets installed 
on the U-boats. Meanwhile laboratory developments 
and small-scale production of homing and direction- 
finding search receivers anticipated a revival of U- 
boat radar. It is possible that submarine radar would 


a At the end of the war German U-boats of high submerged 
speed (10-15 knots for Type XXI, 15-25 knots for the proposed 
Type XXVI) were approaching operational status. They were 
designed to operate effectively when submerged and would have 
been able to stay submerged using Schnorchel to escape radar 
detection, yet retain potent offensive capabilities because of their 
special propulsion for high submerged speed. Conclusions drawn 
from operations involving old-style U-boats may be completely 
inapplicable to these new types. Their development shows, how- 
ever, that the Germans had realized that the other types were 
not satisfactory for completely submerged operation. 




153 


154 


SEARCH RADAR IN RELATION TO ENEMY COUNTERMEASURES 


be a handicap rather than a help if opposed by ade- 
quate airborne listening gear.^ 

3. Radar decoy. Various types of radar decoys or 
false targets have been employed in the hopes of 
drawing the attention of Allied radar search craft 
away from the U-boats themselves. Metallized fabric- 
covered balloons [RDB] and arrays of reasonant di- 
poles on spar buoys (Thetis) have been used, espe- 
cially in the Bay of Biscay. In sufficient numbers they 
might be expected to lead to a serious waste of search 
time and effort, and there has been at least one in- 
stance of a United States surface vessel being tor- 
pedoed while investigating a decoy target. In general, 
however, decoys have not been very successful in con- 
fusing Allied ASV radar search, largely because they 
proved to be too small as radar targets to be detected. 
In addition, careful study of target motion aids the 
radar operator in distinguishing stationary spar 
buoys or wind-blown balloons from true targets. 

The most significant German countermeasures to 
Allied search radar have been twofold— first, the de- 
velopment of intercept receivers and, second, the de- 
velopment of Schnorchel and associated radar cam- 
ouflage. These will now be discussed. 

14.2 the problem of german 
SEARCH RECEIVERS 

For submarines, in particular, employment of an 
effective radar intercept receiver provides a very 
satisfactory countermeasure to search radar. Upon 
receiving a signal of sufficient intensity from the 
enemy radar, the submarine can dive and become 
completely hidden. So long as the intercept receiver 
can be depended on to outrange the enemy radar (as 
it should, in principle, usually be able to do), the 


b United States submarine crews have to some extent shared 
the Germans’ suspicion of aircraft warning radar and convinced 
themselves that the Japanese were homing on SD radar trans- 
missions from a number of individual incidents which seemed 
to indicate such homing. Many of them ceased to use SD radar. 
Statistical data showed, however, that no effective homing was 
taking place as late as December 1944, as can be deduced from 
the table below . 


Aircraft contacts 

Percentage of aircraft 

per 100 days 

(all areas) 

(in Luzon strait) 

that detected subs 

Day 

Night 

Day 

Night 

SD nonusers 84 

23 

12 

10 

SD users 86 

24 

9 

8 


protection offered is complete. Nevertheless, such 
complete protection is rarely achieved in practice. 
(See Chapter 5 in Volume 2B.) It is valuable to de- 
scribe the various stages in the radar versus search re- 
ceiver competition that took place in the U-boat war 
as an example of the problems involved in evaluating 
correctly the effectiveness of countermeasures of this 
type. 

14.2.1 Meter Radar Is Compromised by 
Metox German Search Receiver 

Even as early as August 1941 it had been suggested, 
since aircraft not employing ASV radar were more 
successful than aircraft using ASV in sighting U- 
boats, that U-boats were detecting ASV transmis- 
sions. To obtain evidence of this an experiment was 
carried out by Coastal Command, and aircraft oper- 
ating off Brest observed ASV silence on alternate 
weeks for a brief period. The results of this experi- 
ment, given in Table 1, showed that there was no 
evidence of any disadvantage involved in the use of 
ASV, since aircraft using radar made more contacts 
than those not using radar in the same amount of 
flying. 


Table 1. Contacts with and without radar. 
(Biscay area, September 1941.) 



Radar off 

Radar on 

Flying hours in area 

528 

541 

Contacts: Visual 

3 

51/2 

Radar 


21/2 

Total 

3 

8 


Later intelligence showed that this conclusion was 
indeed correct. From the start of World War II, the 
Germans were fully aware of the possibilities of 
meter ASV radar and had developed their own air- 
borne search equipment, but it was not until the 
summer of 1942 that they concluded that the Allies 
were using radar for U-boat search and initiated a 
hurried program for the development of search re- 
ceivers to detect the radiations. The first equipment 
to be installed on U-boats was the R-600 or Metox 
with a low wavelength limit of 130 cm. It was of the 
heterodyne type, thought to be the only type capable 
of sufficient sensitivity, and so it radiated energy, a 
property which eventually caused its abandonment. 
Nevertheless, it was used with apparent success, and 
the conditions of its introduction and use are of con- 
siderable interest. 


THE PROBLEM OF GERMAN SEARCH RECEIVERS 


155 



JUN JUL AUG SEP OCT NOV DEC 


1942 

Figure 1. Fraction of U-boat transits of Bay of Biscay 

that were sighted. 

The German development of search receivers was 
closely related to the progress of the aircraft anti-U- 
boat offensive in the Bay of Biscay. In June 1942 the 
introduction of night attacks by Leigh Light Wel- 
lingtons and the accompanying sharp rise in the 
number of attacks made on U-boats caused the Ger- 
mans to take some action, and the Metox German 
search receiver [GSR] was the result. Its operational 
success against the British Mk II radar was consider- 
able, as sightings in the Bay transit area were greatly 
reduced. Figure 1 shows that the fraction of U-boat 
transits sighted dropped markedly with the intro- 
duction of Metox after the summer of 1942, although 
the amount of flying done remained approximately 
constant. 

During the period shown in Figure 1 data on the 
effectiveness of radar search were also obtained in 
other regions than the Bay of Biscay, for example, in 
the Trinidad area of the Caribbean Sea Frontier. 


Table 2. Contacts with and without radar. 
(Trinidad area, Aug 1942 -Jan 1943.) 


Month 

Hours of flying 

Contacts 

Effective search 
rate in sq. miles 
per hour 

No radar 

Radar 

No radar 

Radar 

No radar 

Radar 

Aug 

1,511 

851 

11 

6 

526 

507 

Sept 

3,326 

1,650 

8 

15 

93 

351 

Oct 

1,878 

3,164 

6 

21 

120 

248 

Nov 

1,016 

3,714 

0 

9 

0 

149 

Dec 

3,229 

1,200 

6 

2 

106 

95 

Jan 

3,648 

1,544 

4 

2 

97 

114 

Total 

14,608 

12,123 

35 

55 

127 

241 


These data, like those obtained earlier by Coastal 
Command, showed little evidence of any effective use 
of radar listening gear by the U-boats, as shown in 
Table 2. Two conclusions were possible, either that 
the use of the gear was much less effective in the 
Trinidad area than the Bay of Biscay or that the 
change in the Bay was not due to Metox but was, for 
example, a seasonal change. The actual situation 
may have involved more psychological benefit from 
the Metox than physical benefit. The use of Leigh 
Light Wellingtons in the summer of 1942 may have 
scared U-boats into abandoning the policy of sur- 
facing at night even though the number of planes was 
small and surfacing in the daytime was still much 
more dangerous. With the advent of Metox they 
may have returned to the safer policy of nighttime 
surfacing. In the Trinidad area, on the other hand, 
there was no radical change in U-boat tactics except 
for a general increase in caution. 

If this last explanation is taken as the correct one 
it is still, in a sense, fair to say that Metox caused the 
change in Bay of Biscay results in the fall of 1942. It 
would not be safe, however, to conclude that Metox 
was, therefore, a very effective search receiver. The 
actual mechanism linking cause and effect was a 
more subtle one. Whatever the explanation of the 
apparent success of Metox, it was short-lived, for the 
Allies soon introduced S-band (10-cm) radar and the 
effectiveness of air search reached even higher levels 
than before. 

H.2.2 Germans Baffled by S-Band Radar 

Meanwhile, Allied development of airborne S- 
band radar was proceeding, based on the magnetron 
transmitter tube, and it was put into operational 
service early in 1943 as the U.S. SCR-717 and ASG, 
and the British Mk III types. This new wave band 
not only provided immunity from detection by 
Metox but also gave increased ranges of detection on 
U-boats.^' The operational success of this type of gear 
was considerable. Aided by the seasonal upswing in 
aircraft effectiveness normally occurring in the 
spring months because of better weather and more 
daylight hours, S-band radar had much to do with 
the peak of aircraft achievements reached in the 
summer of 1943. 

Evidences of the success of the air war against 

c See Volume 2B, Chapter 5, for detailed discussion of radar 
ranges. 




156 


SEARCH RADAR IN RELATION TO ENEMY COUNTERMEASURES 



1943 


Figure 2. Fraction of U-boat transits of Bay of Biscay 
sighted. 

U-boats at that time are shown in Figures 2 and 3 be- 
low. Figure 2 presents data on the sighting of U-boats 
passing through the Bay of Biscay. The sharp rise iu 
the spring months is very evident. In Figure 3, which 
gives the overall results of the aircraft offensive in 
terms of U-boats sunk per month, there is also a sharp 
rise at this time. S-band radar was by no means the 
only cause of this increase in air effectiveness, but 
there is no doubt that it made a significant contribu- 
tion. The other major factor was the introduction of 
aircraft based on escort carriers for mid-ocean offen- 
sive operations. 

With this upswing in Allied aircraft success, the 
Germans became convinced that Allied aircraft were 
using some new detection device and started a frantic 
activity to identify and counter it. For a time they 
occupied themselves with the idea that it was an in- 
frared detector, since they had tried to develop one 
of their own, and experimented with special paints 
intended to give no infrared reflections. They also 
considered the possibility of a frequency-scanning 
radar and developed a scanning receiver with a cath- 
ode-ray tube presentation. This was of definite ad- 
vantage to the operator, but it still covered only the 
same meter-wave band. The sinkings of U-boats 
continued. 

In desperation they jumped to the conclusion that 
their GSR radiations were being homed on. The 
Metox receiver was outlawed and the Wanz G-1 in- 



Figure 3. U-boats sunk per month by aircraft. 

troduced. This was of an improved design and radi- 
ated much less power. However, the almost patho- 
logical fear of radiation which had been bred in the 
minds of U-boat captains prevented them from 
trusting it. Continued sinkings and skepticism of the 
technical advantages kept it from being used. 

Next the German scientists turned to the much 
less sensitive crystal detector-receiver, which was en- 
tirely free from radiation, and produced the “Bor- 
kum.” This was a broad-band intercept receiver 
which covered the 7-300-cm band. Neither it nor the 
Wanz was effective against the new Allied radar, 
however. 

Finally, in September 1943, the U-boat command 
realized that 10-cm radar was being used against 
them. The “Rotterdam Gerat,” a British H 2 S radar 
working in the 10-cm band, had been captured intact 
at Rotterdam by the German Air Force in March 
1943, and German scientists had soon determined its 
characteristics. How the 6-month delay from March 
to September occurred is unexplained. It was a sig- 
nificant time factor in the U-boat war. A further 
delay of about 6 months intervened before the first 
really effective S-band receivers became operational 
in April 1944. During this interval the frantic experi- 
mentings of the German Technical Service became 
evident in such incidents as the patrol of the U-406 
carrying one of their best GSR experts. Dr. Greven, 
and his staff, with a full complement of experimental 


THE PROBLEM OF GERMAN SEARCH RECEIVERS 


157 


search receivers. The U-406 was sunk, and other ex- 
perimental patrols also had short careers. 

The drop in results of the Allied air effort at the 
end of the summer of 1943, which is shown in both 
Figure 2 and Figure 3, was only in small part due to 
GSR developments. The U-boats simply adopted an 
ultraconservative policy of maximum submergence 
and rarely exposed themselves to air attack. In pass- 
ing through the Bay of Biscay they crept along the 
Spanish coast, in regions inaccessible from Britain, 
and surfaced as little as they could. With such tactics 
U-boat effectiveness was very low, but they gained 
respite from air attack. 

14.2.3 Naxos Search Receiver Covers 
S-Band 

Out of this confusion finally came the “Naxos” in- 
tercept receiver covering the 8-12-cm band. The first 
models introduced in the fall of 1943 were crude 
portable units mounted on a stick and carried up 
through the conning tower upon surfacing. The 
range was short, because of the crystal detector prin- 
ciple, the broad-band coverage, and the small, non- 
directional antenna: estimates of range from pris- 
oner-of-war reports were 8 to 10 miles. 

Allied reaction to intelligence reports about Naxos 
as early as December 1943 brought the fear that 
S-band radar was compromised. Even earlier than 
this (November 1942) “disappearing contacts” had 
led many to assume compromise long before 10-cm 
search receivers were thought of by the Germans. A 
serious morale problem developed among Allied 
ASV flyers with this news and the drop in U-boat 
contacts. Radar was turned off completely in several 
squadrons where its use could only have resulted in 
more numerous contacts. Tactics were improvised to 
salvage some usefulness for the radars on the assump- 
tion that the GSR would outrange the radar (an 
assumption that was largely false). Some of them are 
listed below. 

1. Prohibition of special radar procedure during 
the approach, such as “searchlighting” the target, 
sector scan, or change of scan rate, since such changes 
would indicate to a GSR operator that radar contact 
had been made, and the U-boat could then take 
evasive action. (It was considered unlikely that 
U-boats would dive immediately on receiving a sig- 
nal on GSR.) 

2. Attenuators, such as “Vixen,” were initiated to 


cause a slow and steady decrease in transmitted power 
as range closed and so to confuse the GSR operator. 
In order to use Vixen successfully, the contact must 
be made at a range of 15 miles or greater and the 
cycle started soon after. Since this is greater than the 
average radar range under many conditions, it could 
only be used for less than half the contacts. Produc- 
tion was slow and installation slower, with the result 
that Vixen had no operational opportunity for justi- 
fying the effort spent in its development. 

3. An interim tactic of “tilt-beam” approach was 
proposed to reduce signal intensity as range was 
closed by tilting the radar beam up off the target. 
This requires unusual skill and cooperation between 
pilot and radar operator to be effective, and its value 
has never been adequately proved. 

4. Almost in desperation the tactic of turning the 
spinner aft (for the 360-degree scanning radars) and 
approaching by dead reckoning was suggested. The 
chances of a successful navigational approach are 
small, however, as compared with radar homing on 
the target, and this proposal was not very promising. 

A serious error in some Allied thinking at this 
phase consisted of overestimating the capabilities 
and efficiency of the Naxos GSR. It was felt that such 
a search receiver would completely nullify the effec- 
tiveness of S-band radar, but such did not prove to 
be the case. The data presented in Table 3 show the 
effectiveness of night flying during the period Oc- 
tober 1943 to January 1944 in the Bay of Biscay (the 
region in which use of GSR might be expected to 
cause the largest reduction). 

If we consider that the discrepancy between ex- 
pectation and results obtained is entirely due to 


Table 3. Results of S-band radar night flying. 
(Bay of Biscay, October 1943-January 1944.) 



Expected 


Disappearing 


contacts 


contacts 

Type of aircraft 

on basis of 


probably 

and radar 

previous months 

Sightings 

on U-boats 

Wellingtons, 




British Mk III 

62 

15 

23 

Halifaxes, 




British Mk III 

32 

18 

15 

Liberators, 




ASG, searchlights 30 

12 

14 

Liberators, 




ASG, no searchlights 25 

1* 

13 

Total 

149 

46 

65 


* This poor showing probably due to lack of experience. 


158 


SEARCH RADAR IN RELATION TO ENEMY COUNTERMEASURES 


GSR, we would conclude that it enabled 25 per cent 
of the U-boats that would normally have been con- 
tacted to escape detection altogether, 44 per cent to 
dive after being contacted by radar but before being 
sighted visually, leaving 31 per cent that were still 
sighted, even with GSR. At the most, then, it can be 
concluded that a reduction of 69 per cent was caused. 
As an overall average, day and night, including other 
areas, the reduction would be much less, possibly 25 
to 50 per cent. Since the sweep rate in S-band radar 
search is under most conditions several times that for 
visual, the use of radar was still imperative. 

Efforts were made, therefore, to revive the con- 
fidence in radar and keep it in operation. Gontacts by 
S-band radar continued to be made, and it became 
evident that the Germans did not have much confi- 
dence in the effectiveness of their search receivers, 
nor did they use them consistently. In the coming 
months they were to become more and more com- 
mitted to the use of Schnorchel rather than search 
receivers to defeat Allied radar. 

14.2.4 X-Band Radar and Tunis GSR 

Since an S-band search receiver could, in principle, 
be highly effective, the use of radar of an even higher 
frequency was an obvious next step as a countermeas- 
ure to Naxos. Development and allocations of X- 
band (3-cm) equipments even preceded the advent of 
Naxos and were further stimulated by the problem 
it presented. However, the Germans were not caught 
napping this time. An H 2 X blind-bombing aircraft 
was lost over Berlin in January 1944, and from the 
damaged remains the Germans learned of the new 
frequency band. It was assumed that this frequency 
would also be applied to ASV radar, and the develop- 
ment of X-band intercept receivers was started before 
use of the X-band radar by the Allies in U-boat 
search had produced many results. A well-designed 
receiver was developed, known as Tunis, which con- 
sisted of two antennas, the Mucke horn for X-band, 
and the Cuba la (Fliege) dipole and parabolic reflec- 
tor for S-band, and installations started in the late 
spring of 1944. Installations seem to have been com- 
pleted during the period of inactivity following the 
withdrawal to Norwegian and German bases, but the 
operational use of Tunis was not great, because of the 
reliance placed on Schnorchel from that time on. 

The chief feature of Tunis was the directional an- 
tennas which gave increased sensitivity and range. To 


obtain full coverage, the antennas were rotated 
manually at about 2 rpm. Allied tests on captured 
specimens of this gear showed good performance 
with a range of about 15 miles on an aircraft at 100 
ft increasing to 40 miles or more at altitudes in excess 
of 1000 ft. Ranges obtained in operational use were 
undoubtedly somewhat shorter, but Tunis was ap- 
parently an effective search receiver. 

The outstanding tactic proposed for Allied use 
against Tunis was intermittent radar operation. A 
schedule of two or three radar scans at intervals of 1 
to 2 minutes for a narrow-beam radar will point the 
beam on target for only a small fraction of the time. 
Since the sweeping GSR beam is only occasionally 
pointed at the aircraft, and since a coincidence of 
these events is necessary for the GSR to achieve a 
detection, the probability of detection by GSR can 
be made rather small. Discussion of this procedure is 
given in Volume 2B, Chapter 5. Since it was not used 
to a significant extent in operations, it will not be 
discussed further here. 

14 3 USE OF SCHNORCHEL AND 
CAMOUFLAGE 

Apparently dissatisfied with search receivers as a 
means of achieving immunity from radar detection, 
the U-boat Command turned to more drastic meas- 
ures during the last year of the war. The develop- 
ment of Schnorchel was carried out during the latter 
months of 1943 and fitting of the equipment to 
U-boats began early in 1944. Using Schnorchel for 
Diesel intake and exhaust, the U-boat could run at 
periscope depth with only a small Schnorchel head 
showing above the surface, yet accomplish the venti- 
lation of the boat and recharge of batteries that pre- 
viously had required surfaced operations. Starting in 
the spring of 1944, U-boats spent very little time on 
the surface, employing Schnorchel or electric propul- 
sion almost exclusively. 

Original Schnorchel gave reduced radar echoes 
merely because of their small size. As a result the 
average range of detection is reduced to about a 
third of that on a surfaced U-boat. In addition, the 
echo is frequently too small for detection until it has 
entered the area of sea returns which mask it quite 
effectively, and many potential Schnorchel contacts 
are missed completely for this reason. Even without 
camouflage, a Schnorchel is a difficult target to de- 
tect by radar. Not content with this state of affairs. 


USE OF SCHNORCHEL AND CAMOUFLAGE 


159 


the Germans developed nonreflective coatings for ap- 
plication to Schnorchel which still further reduced 
the echo from it. Fortunately this camouflage reached 
operational use only a few months before the end of 
World War 11. 


Radar Detection of Schnorchel 


In order to evaluate the performance of radar in 
detecting Schnorchel numerous trials have been con- 
ducted with dummy or mockup Schnorchel targets. 
It is not intended to present a summary of them here. 
The values quoted in Table 4 are typical. The figures 


Table 4. Test results on Schnorchel detection. 
(ASWDU trials.) 



Type radar 

AN/APS-15 

ASG 

Average range on surfaced sub (miles) 

32 

19 

Average range on Schnorchel (miles) 

10.5 

4.1 

% runs on which contact made on 



Schnorchel 



Sea states 1 and 2 

82 

67 

Sea states 3 and 4 

55 

32 


show a serious reduction in both range and reliability 
of detection as compared with that on a surfaced 
U-boat. Depending on the radar set involved and the 
sea state, the reduction in sweep rate varies from 75 
per cent to 95 per cent, even though the position of 
the Schnorchel was approximately known in these 
trials, which would make detection unrealistically 
easy. 

Operational results have been even more discour- 
aging. For the period November 1944 to March 1945 
an analysis was made of a region near the British 
Isles which contained about 0.25 U-boat per 1000 
sq miles. Assuming that they had been Schnorchel- 
ing about a quarter of the time, the expected number 
of sightings made by the 19,360 flying hours done in 
the area would be: 


Number of sightings = area swept x density 

of Schnorchels 


= 19,360 X Q X 

= 1.2Q. 


I X 0.25 
1,000 


The total number of sightings and disappearing 
radar contacts made at night was 16, so that 





13 sq miles per hour. (1) 


The sweep width is then about 1/10 of a mile, only 
about 1 per cent of the value on a surfaced U-boat. 
In operations, therefore, Schnorchel has been very 
successful in countering radar detection. In the day- 
time, when visual sightings are also possible, its im- 
munity from contact is not quite so great. The sweep 
width has been estimated at 0.6 mile. 

14.3.2 Countermeasures to Schnorchel 

A number of countermeasures to Schnorchel have 
been employed, among them the following. 

1. Modifications in tactical doctrine to match the 
decreased radar sweep width (see Volume 2B). This is 
not a universal scale factor, but must be analyzed for 
each search plan. For example, the shorter aircraft 
radar range is partially compensated for by the de- 
creased submarine speed. 

2. Radar modifications to improve the efficiency of 
contact, such as sea return discriminator circuits. 

3. New radar developments of high-power, nar- 
row-beam, short-pulse equipments designed to pro- 
vide better resolution in search for small targets. 

These have, however, by no means solved the prob- 
lem, and Schnorchel remains a very difficult object to 
detect. In the overall picture of ASW, surface craft 
have played a very important part in combatting 
Schnorchel operations. The decreased speed and mo- 
bility of a Schnorcheling submarine require it to 
operate in relatively restricted waters and focal areas. 
In such conditions search and counteroffensive oper- 
ations by surface craft have good chances of success. 
As mentioned in Chapter 13, the use of sono-buoys 
for detection of Schnorchel has promise. 


14.3.3 German Camouflage Developments 

The general problem of radar camouflage had 
been under intensive study in Germany since June 
1943, but the decision to apply absorptive coatings to 
Schnorchels was not made until the fall of 1944. 
Plans were made and put into effect at this time to 
provide microwave protection for all U-boats, and it 
is estimated that 100 to 150 craft were actually fitted 
with coated Schnorchels before the end of March 
1945. 

Two types of coatings were employed, the Jau- 
mann and Wesch absorbers. The Jaumann absorber 
was made up of spaced graduated layers of semicon- 




160 


SEARCH RADAR IN RELATION TO ENEMY COUNTERMEASURES 


ducting paper. The Wesch absorber was made up of 
rubber mat containing a high percentage of iron 
powder. 

The Jaumann absorber was the more effective, and 
German reports appear to be reliable in that the 
range of detection on a Schnorchel coated with it was 
only 15 per cent of the range on the uncoated Schnor- 
chel. One of its disadvantages, however, was that it 
had to be preformed and could be used only on flat 
and cylindrical surfaces. The absorptive properties of 
the Wesch absorber were not so good. 

It may be concluded, therefore, that the use of such 
absorbers would make Schnorchel practically safe 
from detection by 10-cm (and also 3-cm) radar. The 
Germans were concerned with the possibility of 
danger from radar of longer wavelengths, but it is 
doubtful that radar of this type would be very effec- 
tive against such small targets. Use of a variety of 
frequencies would counter the effectiveness of these 
absorbers to some extent, until absorbers are devel- 
oped to cover the whole microwave band. 


144 CONCLUSIONS 

It can thus be seen that this succession of measures 
and countermeasures, tactical and otherwise, repre- 
sented, in the main, a series of concessions by the 
U-boats to the effectiveness of radar-equipped air- 
craft. The result has been a great reduction in the 
mobility and attack potential of the U-boats, which, 
however, has gained them considerable immunity 
from airborne radar search. This situation was recog- 
nized very clearly by the Germans, and their large- 
scale program for the development and introduction 
of new types of U-boats of increased underwater 
speed and endurance was an attempt to overcome 
these limitations. Whether future submarines, which 
will probably be forced to operate almost totally sub- 
merged (or on a Schnorchel), can achieve the success 
which submarines had in World War II is a problem 
for speculative analysis beyond the scope of this dis- 
cussion. New technical developments make it seem 
quite possible, however. 





Chapter 15 

COUNTERMEASURES TO THE GERMAN 
ACOUSTIC TORPEDO 


151 INTRODUCTION 

D uring the last 20 months of World War II Ger- 
man U-boats made operational use of acoustic 
torpedoes, which were first employed in September 
1943. The general performance of the torpedo is as 
follows: It is fired in the normal manner, that is, 
under gyro control on a collision course. When the 
torpedo comes sufficiently close to a noise source, 
such as the ship’s propellers, it hears the noise and 
“homes” on it, i.e., turns towards the source and at- 
tempts to keep headed towards it until a hit occurs. 
The torpedo need merely come within homing 
range of the ship to have a high probability of hit- 
ting. In effect it makes for itself a larger and more 
circular target than that of an ordinary “straight- 
run” torpedo. The louder the ship (in the frequency 
band received by the torpedo), the greater will be the 
homing range and consequently the effectiveness of 
the weapon. 

15.1.1 Types of Countermeasures 

The simplest of the several possible countermeas- 
ures to such a torpedo— slowing the target— is sug- 
gested by the fact that a ship’s sound output increases 
rapidly with speed. A ship is so much quieter at 7 
knots than at 15 knots that the homing range is only 
about one-fifth that at the higher speed, and the effec- 
tive target size is correspondingly reduced. Slowing 
considerably increases vulnerability to torpedoes of 
other types, however. Other means of quieting the 
ship’s propellers, such as masking them with bubbles, 
have at best halved the homing range in tests to date. 

Slowing the ship is an example of a tactical coun- 
termeasure since no new gear is required for its in- 
troduction. A materiel countermeasure, on the other 
hand, is one which does involve the use of special 
equipment. Another type of tactical countermeas- 
ure is possible if the target ship has sufficient warn- 
ing as to when and where the torpedo is fired and if 
the torpedo’s homing range is not too great. A radi- 
cal maneuver may be made which will keep the ship 
out of listening range of the torpedo. This has been 


called a “step-aside” when used by an antisubmarine 
vessel attempting to approach the submarine for an 
attack. It is useful only when contact has been made 
at long range and hampers the ship considerably in 
its attempt to get in to sonar contact. 

The third tactical countermeasure is to maintain 
a speed greater than that of the torpedo and is there- 
fore unavailable to many ships. A torpedo homing on 
a target faster than itself will usually be left behind. 
Its listening sensitivity is limited by its own self 
noise,^ which in turn increases rapidly with its speed. 
Thus some compromise is required between speed 
and homing range, and an acoustic torpedo will usu- 
ally be slower than its straight-running counterpart. 
The ship speed required to outrun the German tor- 
pedo was initially considered to be about 20 knots; 
this estimate was later raised to 25 knots, but final 
information indicated that 20 knots was adequate 
after all. Continual rumors of a 30-knot modification 
of the torpedo cast doubts on these figures, however. 

In addition to the above tactical countermeasures, 
there are several possible materiel countermeasures. 
These usually involve noisemakers [NM’s], which are 
small devices, yet several times louder than a ship at 
the frequencies received by the torpedo. The NM’s 
must be placed so as to attract the torpedo on its 
initial aproach and thereafter prevent its getting into 
a position to home on the ship. By the end of 1943 
escort vessels were equipped to tow one or two noise- 
makers about 200 yd astern. Equipment of this type 
was the standard materiel countermeasure employed 
during the remainder of World War II. 

The evaluation of such countermeasures would be 
much simplified and shipboard morale much im- 
proved if some indication were given when a tor- 
pedo has been successfully countered. Development 
work was done on several devices to detonate the 
torpedo before it reached the ship. These included 
nets, rotating barriers, explosive streamers, and mag- 
netic fields set up by trailing electric cables. They 

a Noise generated by the torpedo itself, particularly its pro- 
pellers, makes a substantial contribution to the background 
noise heard by the acoustic mechanism. This contribution is 
called self noise. 


161 


162 


COUNTERMEASURES TO THE GERMAN ACOUSTIC TORPEDO 


were simplest and most effective when used with 
towed NM’s. However, they were never issued be- 
cause they were fragile, difficult to handle, uncertain 
in effectiveness, and seriously impeded the ship’s 
maneuvering. 

A towed NM alone interferes considerably with 
the antisubmarine vessel’s offensive. The towline 
restricts the maneuverability of the ship, and the 
sound generated interferes with sonar and gives the 
submarine early hydrophone warning. To get around 
these disadvantages consideration was given to ex- 
pendable noisemakers projected from the ship or 
dropped astern. By 1945 such noisemakers had been 
produced for masking U. S. submarines from enemy 
sound gear, and their practicability as a torpedo 
countermeasure had been tested. As long as there is 
danger of a torpedo being fired, an NM must be pro- 
jected at least once every 90 sec. This expenditure is 
feasible only when a submarine is known to be close. 
It has long been envisaged that the ultimate counter- 
measure to acoustic torpedoes would incorporate a 
detector which would give sufficient warning of an 
approaching torpedo to investigate the successful use 
of expendable NM’s. Such a detector is not yet 
available. 

Scope of Further Discussion 

The bulk of this chapter deals with the evaluation 
of towed NM’s, since this type of countermeasure was 
of primary importance during World War II. The 
effectiveness of such a countermeasure depends on 
the mode of operation of the torpedo, and, therefore, 
the discussion deals chronologically with the evalua- 
tion in the light of changing information concern- 
ing the nature of the torpedo. 

The most important information to be gleaned 
from operational data is also included. The data are 
discouraging since in most cases only those torpe- 
does which hit ships are detected. In addition it is 
difficult to decide which torpedoes were acoustically 
controlled and this makes interpretation of the data 
uncertain. It is hoped that these difficulties can even- 
tually be overcome by analysis of German Admiralty 
records, which should give further information on 
the operational effectiveness of Allied noisemakers. 

Expendable NM’s did not see operational use and 
will not be discussed further. They must be kept in 
mind, however, as having possible future usefulness. 

Detailed evaluation of tactical countermeasures is 


also omitted. It is now felt that high speed is the only 
tactical countermeasure likely to be very effective, 
and it can rarely be used. 

15.2 DEVELOPMENT OF TOWED 
NOISEMAKERS 

15.2.1 German Introduction of Acoustic 
Torpedoes 

By early 1943 intelligence information indicated 
that the Germans had an acoustic torpedo in produc- 
tion and that successful running tests had been made 
the previous summer. Accordingly, the Allied study 
of countermeasures, which had been following along 
with the development of similar torpedoes, was put 
on a high priority. Soon the different possibilities 
mentioned in the Introduction had all been given 
some consideration. 

An excellent towed NM was already in use for 
sweeping acoustic mines. This device consists of two 
steel bars, clamped parallel close together, which are 
pulled through the water in the direction perpen- 
dicular to the plane containing their axes. The water 
flowing between them causes the bars to strike against 
each other and give off much more acoustic energy 
than does a ship at all frequencies greater than 1 kc.^ 
Both Great Britain and the United States proceeded 
to modify noisemakers of this type so that they could 
be towed at an adequate speed (at least 10 knots) and 
produce good sound output for a life of several hours. 
The parallel-bar type of noisemaker has subsequently 
been in general use by both navies. 

By May 1943 the British had information indicat- 
ing that the German torpedo homed at 20 knots with 
a minimum turning radius of about 100 yd and that 
its homing range might be as great as 1000 yd. They 
also suggested that the torpedo might be “forward- 
listening,” in other words, that the hydrophones 
might be very insensitive to any sound approaching 
from near the torpedo’s stern (in order to reduce the 
self noise from its propellers). 

From this information the British concluded that 
two NM’s should be towed 200 yd astern and sep- 
arated by about 100 yd. As will be shown later, the 

b An acoustic torpedo could not be sensitive to a lower fre- 
quency because a hydrophone system small enough to fit in a 
torpedo would not be sufficiently directional, that is, could not 
tell the side on which a noise was heard. 


DEVELOPMENT OF TOWED NOISEMAKERS 


163 


torpedo is very likely to overtake a towed noisemaker 
from the rear at some time in its gyrations and to pass 
over the NM and head toward the ship. With a for- 
ward-listening torpedo there might be considerable 
danger that the torpedo would escape from a single 
NM at this time, hearing and overtaking the ship 
while keeping the NM in the insensitive arc astern. 
If, however, as it left one NM behind, the torpedo 
found another on its beam, this second NM could be 
relied on to take control. The simplest way to get this 
protection was to tow two noisemakers abreast, suffi- 
ciently separated to assure that whenever the ship 
was ahead of the torpedo there was also an NM 
within 110 degrees of the torpedo’s bow. To provide 
separation the British began developing a diverter 
with the necessary rudders to tow an NM fastened 
below it at the required distance out of the ship’s 
wake. They already had a parallel-pipe noisemaker 
with a sound output 20 db over that of a ship and an 
endurance of 25 hr at 12 knots. The name FOXER 
was given to the complete set of this noisemaking 
gear, including two NM’s, two diverters, and the tow- 
ing hawsers. 

In the United States a parallel-bar noisemaker des- 
ignated FXR was in production by July 1943. Pre- 
liminary theoretical studies had been made of the be- 
havior of an acoustic torpedo under the influence of 
a ship and an FXR 500 ft astern. The tacit assump- 
tion was made that the torpedo could listen equally 
well in all directions. It was concluded that one NM 
could maintain control of the torpedo once the tor- 
pedo had reached it and that the only danger arose 
from torpedoes fired from directly ahead which were 
drawn in to, or very close to, the ship while homing 
on the FXR.<= 

During the summer some prisoner of war informa- 
tion^ (later proved false) indicated that the hydro- 
phones were along the torpedo’s sides. This tended to 
substantiate the all-around listening theory and the 
belief in the adequacy of one noisemaker. 

Prisoners of war also stated that the torpedo was 
for use primarily against escort vessels, enabling the 
U-boat to disrupt a convoy’s defenses. This possibil- 
ity had not occurred to the Allies, probably because 


c Trajectories of this type are discussed on page 168. 
d Much of the early information concerning the German 
acoustic torpedo was obtained from interrogation of prisoners 
of war. Unfortunately they did not, in general, know anything 
of its actual operation, so that it was necessary to deduce its 
method of operation from fragmentary observations which they 
had happened to make of its construction and use. 


the screening vessels had never been a major target,® 
and because it was considered unlikely that the self 
noise could be reduced enough to allow a speed as 
great as 20 knots. Escort vessels now seemed the most 
logical targets for an acoustic torpedo, however. 
Their speed, maneuverability, small size, and the 
frequency with which they were bearing down on the 
U-boat and thus presenting a very small bow aspect, 
all tended to make escorts very poor targets for ordi- 
nary torpedoes. With a homing torpedo, however, all 
these disadvantages were cancelled; the higher speed 
gave a louder, and therefore larger target, while bow 
shots were possibly preferable to beam ones. 

More effort was, therefore, expended on parallel- 
bar NM’s suitable for higher speeds. The British ran 
into more difficulties with their cumbersome divert- 
ers, but the possibility of getting such gear used suc- 
cessfully seemed greater when it was to be handled by 
naval crews than by merchant vessel crews. 

Such was the situation when the Germans were first 
ready to use their acoustic torpedo against the Allies. 
It was introduced on September 20-22, 1943, during 
the first full-scale North Atlantic operation since 
May, namely the wolf-pack attack on Convoys ONS 
18 and ON 202. Three escorts and six merchant ships 
were sunk and an escort damaged; acoustic torpedoes 
accounted at least for the damaged escort and two of 
the other escorts. In contrast a German War Order 
claimed 12 escorts sunk, 3 damaged, and 9 merchant 
ships sunk, which indicates a high expenditure of 
torpedoes. Nevertheless, the torpedo proved itself a 
threat which might have been much more effective 
had the enemy been able to get more than two U- 
boats in contact with the convoy at one time. As it 
was, the potentialities of the torpedo caused the 
Allies extremely grave concern. 

On September 23 the Admiralty issued a compre- 
hensive appreciation of the use of the German acous- 
tic torpedo. This included a review of the informa- 
tion available on the torpedo, a preliminary analysis 
of the first attacks (1 and 2 days old), tactical counter- 
measures, and a status report on FOXER, which was 
to be issued although it was hard to stream, short in 
endurance, hampering to maneuvers, and difficult to 
produce. 

In the United States an extensive study of the 
countermeasure problem was begun. On the one 
hand reports on the torpedo’s characteristics were 

e Thirty-nine warships were sunk by U-boats between Janu- 
ary 1942 and August 1943 in contrast to 1541 merchant ships. 


164 


COUNTERMEASURES TO THE GERMAN ACOUSTIC TORPEDO 


collected and evaluated; on the other, development 
of a variety of noiseinaking gadgets was undertaken. 
One of the early decisions was to discourage the de- 
velopment of impulsive noise sources with bursts 
less frequent than about 20 per second; these in- 
cluded rapid machine gun fire into the water and the 
grenade noisemaker composed of a series of explosive 
caps. It was felt that the torpedo probably would not 
respond to such a source, whereas if it did, a relatively 
steady source such as a ship would be able to over- 
ride it even when providing a much weaker rms (or 
average) signal. Such discrimination would occur 
with the time constants which were most natural for 
a torpedo’s amplifying and control circuits. This 
would not only be a good anticountermeasure device 
but would also (and more important) discriminate 
against the irregular peaks which are among the most 
undesirable features of a torpedo’s background noise. 

It soon became apparent, however, that a reliable 
evaluation of the performance of any particular 
countermeasure required a detailed understanding 
of just how the torpedo functioned. The two differ- 
ent assumptions which have been mentioned— for- 
ward-listening and all-around listening— lead to 
quite different NM arrangements. A careful analysis 
of the probable behavior of acoustic torpedoes was, 
therefore, undertaken. Its salient features are out- 
lined in the following sections. 

15.2.2 Torpedo Trajectory Analysis 

I'he primary aim of a study of torpedo behavior is 
to determine the path which the torpedo follows— 
its trajectory— under the conditions that are of inter- 
est. In order to do this a means must be developed 
for rapidly predicting a torpedo’s behavior in the 
presence of a particular NM system and under a par- 
ticular set of assumptions as to an unpleasantly large 
number of torpedo characteristics, as will be appar- 
ent from the following discussion. Of obvious im- 
portance is the torpedo speed. It determines the ship 
speed necessary to outrun the torpedo. The torpedo 
trajectory can be expressed as a function of the ratio 
of the ship’s speed to that of the torpedo. 

Under the influence of a single noise source the 
torpedo can be assumed (for a first approximation) to 
attempt always to steer towards the source. This re- 
sults in a straight path when the target is stationary, 
e.g., an expendable NM. With the target at constant 
speed on a straight course the pursuit curve is a 




Figure 1. Typical tractrix. (.A) In true space. (B) In 
relative space. 

tractrix. Except for an adjustment in scale any trac- 
trix is like that shown in Figure 1. With an (x,y) co- 
ordinate system centered on the target and the target 
heading in the positive x direction, the relative tra- 
jectory is given by equation (1). 

-mzm » 

or more simply, 

y = yo(^tan^J , x = y cot 9 

where k = ratio of target speed to torpedo speed, 

y = distance at which the torpedo first passes 
abeam of the target, 

6 = torpedo’s bearing relative to the target. 

In stationary coordinates whose origin is the target’s 
position at the time the torpedo is first abeam of it 
(at distance yo) target’s positions are given by 
equation (2), the tractrix, by equation (3). 

X = vt, y = 0. (2) 

When dealing with targets moving together, such as 
a ship and towed noisemakers, it is simplest to draw 
trajectories relative to this moving system. For each 


SHIP MOTION ► 

Figure 2. Course of torpedo when circling. (A) Fixed 
coordinates. (B) Relative coordinates (speed ratio = k 
= 1 / 2 I ship motion _^). 




DEVELOPMENT OF TOWED NOISEMAKERS 


165 


-25 


-20 -15 -5 -10 -10 -5 -15 -20 -25 


f'M 

'v f>r 

ml 


— ; 

■ * \ 

( f 1 

/ 

— ^ \ 

— 

y - 



PORT PATTERN 

u STARBOARD PATTERN 


-5 


-10 


-15 


-20 


-25 


-5 


-10 


-15 


-20 


-25 


TORPEDO 

AXIS 

Figuri: 3. Typical sensitivity pattern. 


value of k that it is desired to consider, a template 
sheet can be prepared, covered with a family of 
tractrices— calculated from equation (1)— so that by 
interpolation a pursuit course can be drawn from 
any point in to the center. For any specific problem 
the trajectories can be traced from these templates. 

A torpedo cannot always follow a tractrix, how- 
ever, because its minimum turning radius may pre- 
vent it from making the sharp turn required. If the 
torpedo turns as sharply as possible when trying to 
get headed towards a noise source,^ its path in relative 
space will be a cycloid rather than a tractrix. A typi- 


f Information in the fall of 1943 continued to confirm the fact 
that the enemy had enlarged the rudders of their G7e torpedo 
to give the acoustic torpedo a minimum turning radius of 80 to 
100 m. 


cal cycloid is shown in Figure 2. Templates can be 
drawn for cycloids in the same way as f6r tractrices as 
an aid in drawing the relative diagrams. 

The behavior of the torpedo depends upon acous- 
tical and electronic features as well as speed and turn- 
ing radius. In the fall of 1943 the exact characteristics 
of the German torpedo were unknown, but it was 
considered probable that a listening system was em- 
ployeds and that the following general scheme of 
operation was used in order to determine from which 
side of the torpedo the sound arrived. 

g A listening arrangement would be much simpler than one 
involving echo ranging and less subject to failure. It would 
probably have a greater homing distance. A torpedo would have 
little use for range information. The enemy had had much more 
experience with passive detection equipment. Most important, 
none of the intelligence information suggested echo ranging. 


166 


COUNTERMEASURES TO THE GERMAN ACOUSTIC TORPEDO 


The rudder is controlled by the ratio of the volt- 
ages in two electric channels. If a steady sound source 
is moved about the torpedo at a constant range, the 
voltage in the port channel is much greater when the 
source is within some range of bearings on the tor- 
pedo’s port side than when anywhere on the star- 
board side. Similarly, the starboard channel is most 
sensitive to signals from the starboard side. 

In determining a trajectory, it is necessary to find 
out for each torpedo position just what are the rela- 
tive voltages in each channel due to each noise source. 
To facilitate this another set of templates are drawn, 
one for each assumed sensitivity pattern. Each tem- 
plate is a transparent geographic plot to be centered 
on the torpedo position with the torpedo axis 
marked. By one curve it shows the locus of all points 
at which a standard (arbitrary) noise source would 
create a given voltage in the port channel and, by 
another, the same for starboard channel. Parallel 
lobes are drawn corresponding to sources each an 
integer number of decibels louder or softer than the 
standard. A typical sensitivity pattern of this sort is 
shown in Figure 3. 

A standard noise source at point X in Figure 3 
would give a signal of —12 db in the port channel and 
+ 2 db in the starboard. If, at the same time, a source 
10 db louder than the standard (for example) is pres- 
ent at point A, it would give a signal of 8 db in the 
port channel, and —3 db in the starboard. In order 
to obtain the total voltage in each channel with both 
present, the square root of the sum of the squares of 
the individual contributions is taken. This method 
should be sufficiently accurate unless a source is very 
intermittent, e.g., a grenade exploding only ten times 
a second. 

In constructing these templates it is assumed that 
the sound pressure is inversely proportional to the 
range from the source (6 db less per distance 
doubled). This is only an average value of the atten- 
uation found in experiments, but the effects of con- 
siderable fluctuations can usually be shown to be 
negligible. The ranges involved at critical times are 
too small to allow the linear absorption of sound in 
open water to become important. However, there is 
evidence of considerable attenuation in the ship’s 
wake, about 2 db per foot at 24 kc.^^ 

The simplest sensitivity pattern is a circular one. 

h In the initial trajectory studies, an attenuation of 0.1 db per 
foot was taken as a conservative assumption, even at lower 
frequencies. 




Figure 4. Simplified sensitivity patterns. (A) Circular 
pattern. (B) Modified circular pattern. 


The template curves for each channel are semicircles 
on each side of the torpedo axis, as shown in Figure 
4A. Dead zones of no differential are assumed to be 
present, but negligibly small, ahead and astern of the 
torpedo.^ To break away from all-around listening it 
may be assumed that there is a wide angle of zero sen- 
sitivity astern, leaving the lobes as shown in Figure 
4B. 

A more realistic picture may be obtained by using 
another mathematical approximation which recog- 
nizes that the response of any hydrophone system will 
change smoothly with angle. If 9 is the angle off the 
torpedo’s bow, the contours of equal sensitivity can 
be expressed by cos (NO). The sensitivity will then 
be a maximum ahead (except for a negligible dead 
angle) and will decrease gradually as 0 increases, be- 
coming zero for all ^’s greater than TrjZN. A number 
of such patterns are shown in Figure 5. For A = 
there is some listening at all angles except dead 
astern, whereas for N = \ there is no response any- 
where abaft the beam. 

In an actual torpedo each channel may have its 
own set of one or more hydrophones directed toward 
one side. Alternatively the channels may be con- 

i This pattern is essentially that assumed in the earliest 
United States studies mentioned in Section 15.2.1, this chapter. 



Figure 5. Cos (Nd) sensitivity patterns. 


DEVELOPMENT OF TOWED NOISEMAKERS 


167 


nected through phasing circuits to the same, or par- 
tially the same, group of hydrophones all facing for- 
ward. Either system would lead to the same kind of 
pattern. It is reasonable to expect that if an effort is 
made towards forward-listening, the maximum lobe 
for each channel would probably be rather broad 
and centered between 20 and 60 degrees off the bow. 
It is difficult to secure a rear response of either chan- 
nel less than about 30 db below the maximum, no 
matter what the direction of the noise. There can be 
a rather large angle astern where a loud source would 
bring about approximately the same voltage in each 
channel. Accordingly a template can be drawn for a 
“practical” pattern having a maximum sensitivity 45 
degrees off the bow and sensitivity 30 db lower in 
both channels throughout a 100-degree arc astern. i 

This pattern is, in fact, shown in Figure 3. The 
pattern could be made sharper and the fore-aft dis- 
crimination greater if the frequency of maximum 
sensitivity were over 40 kc. This, however, would 
reduce the homing range as a result of the absorption 
of sound in open water, which becomes appreciable 
at such frequencies. 

The rudder position should depend on the decibel 
differential between the two channels, that is, on the 
ratio of their voltages. Any control involving the ab- 
solute magnitudes of the voltages could not handle 
the widely different signal strengths to be expected. 
By the use of the templates and a curve expressing in 
decibel units the square root of the sum of the 
squares relationship, the signals in the two channels 
can be found in decibels above the same standard; 
these values can then be subtracted to give the dif- 
ferential. 

The most plausible simple assumption as to the 
way in which rudder postion depends on the differen- 
tial is a “flip-flop” type of control in which the rudder 
is thrown hard over whenever one channel has a 
given excess (say 3 db or more) and stays over until 
the other channel has the required excess. This flip- 
flop control appears necessary for a forward-listening 
type of torpedo on the basis of the following 
argument. 

Most torpedoes approaching from forward angles 
will miss a ship (with no noisemakers) on the first 
pass because they cannot turn sharply enough to stay 
on a tractrix and possibly because the principal tar- 

j A pattern of this sort was decided upon in the fall of 1943 
on the basis of United States experience in the design of hydro- 
phones and acoustic torpedoes. 


get may actually be collapsing bubbles in the wake 
somewhat astern of the propellers. Having passed the 
ship’s stern, many of these torpedoes will have the 
ship in the sector on their own stern where they can 
get no differential or even any signal over back- 
ground. If, with no differential, the torpedo straight- 
ens out or returns to its original gyro course, it will 
never hear the ship again and be lost astern. On the 
other hand, with flip-flop control the missile will con- 
tinue circling until it again hears the target. The 
same would happen about a single NM towed astern. 
Admittedly the torpedo would stay on its tractrix 
with less weave if on loss of differential it straight- 
ened out or, preferably, had “graduated control,” 
that is, if the amount of rudder depended on the 
differential and thus on the bearing of the target off 
the bow. However, even with flip-flop control the 
weave should not be more than 10-20 degrees, and 
improvement in this would not compensate for the 
failure of so many bow shots. 

The critical differential, designated by D, may 
reasonably be assumed to be 3 db. With a lower value 
there would be too much danger of undesirable rud- 
der actuations. These might result from minor lobes 
in the sensitivity pattern astern, from fluctuations in 
the background noise, or from fluctuations in the 
transmission of the ship’s sound. With these limita- 
tions, D should be as small as possible, because the 
larger D, the less the torpedo’s homing range. As a 
torpedo approaches a ship from a distance under gyro 
control, background sound will initially predom- 
inate, causing approximately the same voltage in 
each channel. If the bearing to the ship is near the 
angle of maximum sensitivity for one channel, the 
voltage in that channel will increase as the ship is ap- 
proached. The torpedo will turn towards the ship 
when the voltage in that channel due to the ship plus 
background is D db greater than that in the other 
channel, which is due to background plus a negli- 
gible contribution from the ship. If a greater D is 
required, the torpedo must approach closer on gyro 
before the ship noise can overcome background. 

The maximum range at which a given ship flips 
the rudder changes greatly with the bearing $ of the 
ship off the torpedo’s bow. The range is zero with the 
ship either dead ahead or (probably) in a large sector 
astern, that is, with at which the sensitivity pat- 
terns for the two channels differ by less than D db. At 
other 0's the range depends chiefly upon the sensi- 
tivity patterns. For any choice of sensitivity pattern. 


168 


COUNTERMEASURES TO THE GERMAN ACOUSTIC TORPEDO 


of D,^ and of ship intensity over background, a so- 
called steering pattern can be drawn plotting the 
rudder actuation range against 0. Such curves are 
useful only when no NM is present. 

The maximum of either lobe of a steering pattern 
gives the maximum range at which a ship can direct 
the torpedo. The value of this range depends on the 
noise output of the ship and on how well the torpe- 
do’s background noise has been suppressed. Neither 
of these quantities can be determined very accurately. 
Even for a given torpedo and a specihc class and 
speed of ship there would be enough variation in 
ship output and in transmission conditions to make 
the range uncertain within a factor of three. It would 
obviously be unwise to base a countermeasure on an 
arbitrary actuation range.^ 

It is now possible to consider briefly the trajectories 
for which the torpedo was primarily designed, 
namely those against an unsuspecting ship. Assuming 
the torpedo to be a modification of the German elec- 
tric G7e, the firing range was probably not over 6000 
yd. Ranges between 1000 and 4000 yd would be the 
most likely. The initial gyro course would be a colli- 
sion course for the ship, possibly for the ship’s stern, 
in order to get the best chance of coming within hom- 
ing range. When the torpedo reaches a point where 
the ship’s noise can overcome background noise (as 
shown by the ship’s stern reaching the appropriate 
contour on the steering pattern centered on the tor- 
pedo), the missile turns toward the ship. If this hap- 
pens at short enough range, the torpedo might hit 
the ship before turning far enough to head at the 
stern, as shown by trajectory A, Figure 6. Alterna- 
tively, assuming flip-flop control, the torpedo will 
turn until the screws have passed through the dead 
angle of small differential on its bow and have 
reached the opposite lobe of the steering pattern 
where the other channel has a D-db excess, causing 
the rudder to flip. With a forward-listening pattern 
and mechanical time constants of the order of 1/10 

k IL was once ihouglu that there was a good chance tliat the 
1 ) rec|uired to take the torpedo off gyro would he greater than 
that needed for the later steering. Using this “gate” would sacri- 
fice some homing range in order to prevent a temporary peak in 
ship noise from taking the torpedo off gyro course and leaving it 
circling at such a distance that the normal ship noise coidd not 
he heard. However, the gate was assumed not to exist since its 
presence was found to he if anything an aid to countermeasure. 

1 Various estimates were made of the actuation range of the 
German torpedo, ranging from 100 to 1000 yd for a typical ship 
target, l.ater tests suggest the higher figures to have hecn the 
better. 



sec, the course reversal should occur with the target 
less than 10 degrees off the bow. By repetition of this 
procedure the torpedo follows a tractrix towards the 
screws with a small weave. Trajectories marked B in 
Figure 6 are examples of such tractrices. 

If the tractrix approaches from dead ahead, the 
torpedo may hit the ship before coming abeam the 
screws. If the tractrix is entered sufficiently far astern, 
the torpedo will follow it all the way in, making a hit 
on the ship’s stern. In the intermediate cases, includ- 
ing most shots fired from forward of the beam, the 
torpedo is not able to keep headed for the screws be- 
cause the radius of curvature of the tractrix becomes 
less than the torpedo’s minimum turning radius. (In 
the (x,y) coordinates of Figure 1 relative to the source 
the loci of such points are two circles tangent to the 


DEVELOPMENT OF TOWED NOISEMAKERS 


160 


x-axis at the origin and of radius y^KR, where R is 
the turning radius of the torpedo.) The torpedo is 
forced to lose ground astern, usually missing the ship 
on the first pass. Even though the ship may now be in 
the dead zone astern of the torpedo, the flip-flop con- 
trol will cause the missile to continue circling for- 
ward. It may hit the ship’s side before it has circled 
far enough to be headed at the screws, as in trajectory 
C, Figure 6. Otherwise it will make another pass at 
the screws, usually on a rear tractrix leading to a stern 
hit, as in trajectory D. In all cases a hit occurs on at 
most the third pass, as in trajectory E. Any trajectory 
can, like those of Figure 6, be drawn relative to the 
ship by tracing from the appropriate tractrix and 
cycloid templates. 

15.2.3 Theoretical Trajectories and 
Noisemakers 

Next to be considered are the trajectories which 
arise when a single noisemaker E db louder than the 
ship is towed under the wake about 200 yd astern. 
The submarine’s firing procedure would be the same 
as for the ship alone, but the torpedo would get on a 
tractrix towards the noisemaker at ranges of a mile or 
more, if F is 12 db or more. I hus any torpedo in 
working order is drawn into the system. The ship’s 
sound cannot influence the torpedo except within 
about 50 yd of the screws, as long as the NM is on its 
forward bearing. Thus the paths to the NM are 
similar to those to a ship by itself. 

A direct hit occurs when the torpedo is drawn into 
the ship while steering towards the NM. Such hits 
result from tractrices which happen to go through the 
ship. These hits are independent of the NM strength 
or distance astern. To have a good chance of making 
such a hit the firing submarine must be directly 
ahead of the ship, not more than about 3 degrees off 
the bow. Conversely, the ship can be made safe from 
them by never heading directly at the submarine.*" 

If the NM is too weak or too far astern, a forward- 
listening torpedo fired from about 10-20 degrees off 
the ship’s bow may make an indirect hit. This occurs 
when the tractrix comes close enough to the ship that 
the torpedo is forced to circle out of the tractrix 
across behind the ship’s stern and then to pursue the 
ship to a stern hit keeping the NM in the dead angle 

m Allowing a margin of safety, a directive was issued specify- 
ing that when possible any submarine be kept more than 10 de- 
grees off the bow of a ship towing FXR. 


astern. Path A in Figure 7 shows a trajectory which 
might lead to an indirect hit. For the following ex- 
planation assume that the torpedo is weaving to- 
wards the NM past the ship’s port side. While turn- 
ing to the left in the weave, the left channel may 
receive a signal from the ship which is within D db 
of that received in the right channel from the NM. 
(This is determined by employing the sensitivity pat- 
tern template as described in Section 15.2.2.) If, as a 
result of the proximity of the screws, this occurs even 
when the NM is at the angle of maximum sensitivity 
for the right channel, the differential D is never ob- 
tained, and the torpedo continues to circle left as 
shown in Figure 7. During the turn the NM’s bearing 
off the torpedo’s bow increases steadily through 
angles of decreasing sensitivity. Meanwhile in the 
dangerous cases the bearings to the screws (on the 
other side) increase less rapidly and the range to the 
ship decreases, all tending to keep the torpedo cir- 
cling. This condition continues if the torpedo has 
space in which to turn sufficiently (about 90 degrees) 
before crossing the wake. Eventually it will be head- 
ing towards the screws with the NM in its dead angle 
astern and will be guided in to a stern hit. If the 



Figure 7. Typical trajectories relative to a 12-kt ship with 
single towed noisemaker. 


70 


COUNTERMEASURES TO THE GERMAN ACOUSTIC TORPEDO 


tractrix is too close to the ship, however, as in Path A' 
of Figure 7, the bearing to the ship increases rapidly 
at about the time of crossing the wake and the NM 
regains control. 

Thus there may be a dangerous group of tractrices 
close enough to the ship for the latter to deflect a tor- 
pedo but far enough away to allow it sufficient cir- 
cling space. The danger of indirect hits can be elim- 
inated by making the NM sufficiently loud to prevent 
the torpedo leaving any tractrix which is far enough 
from the ship to give circling room. Any reduction in 
torpedo turning circle therefore obviously increases 
the danger from such shots. For the cos (I/2 0) patterns 
or “practical” sensitivity patterns, the E required of 
an NM 200 yd astern is about 10 db. 

Torpedoes which have made neither direct nor in- 
direct hits continue to the NM, as shown in Figure 7. 
As with a ship alone, some approach the NM from 
the rear, making a so-called stern chase. The others 
pass astern of it, circling forward with minimum 
turning radius. These shots circle across in front of 
the NM and continue circling, falling way behind 
the NM and entering a stern chase. All trajectories 
lead to a stern chase after at the most four passes at 
the NM. By employing various sensitivity patterns, 
it can be shown that during this preliminary weaving 
about the NM the ship cannot capture the torpedo 
provided the NM is as much as 200 yd astern and is 
able to hold the torpedo after a stern chase. 

The success of a single noisemaker arrangement 
thus depends on its ability to prevent a torpedo 
which has just passed over it on a stern chase from 
continuing on to hit the ship. The most critical 
factor is the torpedo’s sensitivity pattern." There is 
no problem if the torpedo can hear about equally 
well in all directions (as was assumed in the earliest 
United States studies) since the NM, being louder 
and closer, is certain to be heard over the ship and to 
make the torpedo circle to the rear, where it would 
start another futile stern chase. No matter how loud 
the NM, it cannot be depended upon to counter a 
“mathematical” pattern with a sector astern in which 
the sensitivity is zero (no listening whatsoever), such 
as that in Figure 4B. The NM is likely to fall into the 
dead zone and stay there while the ship noise guides 
the torpedo to a hit. 

n Since accurate detailed knowledge of the German acoustic 
torpedoes sensitivity pattern has not been available, there has 
always been doubt on the subject of safety from stern chases. 
This has, in fact, been the crux of the countermeasure problem. 


The cos (1/2^) pattern, however, with the sensitiv- 
ity greater than zero everywhere but right astern, is 
countered by an NM louder than the ship by 10 db or 
more. In this case attenuation of the ship’s noise in 
the wake is sometimes needed to cause the torpedo to 
turn sufficiently for the NM to regain control. With 
an estimated 0.1 db per foot for the attenuation there 
is no chance that a torpedo in the wake and back 
near the NM could hear the ship. This protection is 
not diminished if the NM should happen to be to 
one side of the wake rather than underneath it. 

The more realistic practical sensitivity pattern 
gives new assurance of the value of a single NM. Hav- 
ing passed over a sufficiently loud NM on a stern 
chase, a torpedo with a practical pattern receives so 
much signal in both channels from the NM that the 
additional ship sound in one channel cannot provide 
the necessary differential to flip the rudder. With flip- 
flop control the torpedo stays in the circle it hap- 
pened to be in as it crossed the NM until it falls way 
behind. For an NM towed 570 ft astern an output of 
12 db above the ship can be shown to be adequate. 
The required output decreases slightly with in- 
creased ship speed because the torpedo’s distance of 
closest approach to the ship is increased. 

On the basis of early recommendations FXR was 
being prepared during the fall of 1943 with 200 yd of 
cable. The cable sag resulted in a towing distance of 
570 ft. The completed trajectory study, made follow- 
ing the above outline, revealed no reason for chang- 
ing this. A longer tow would require a higher output 
to prevent indirect hits and would increase the diffi- 
culties of handling the gear. A shorter tow would 
require a higher output to prevent stern chase hits. 
It would have been nice to tow the NM beyond the 
homing radius of the torpedo on the ship. This was 
precluded, however, by the uncertainty of this radius 
and by the greater NM output required to prevent 
indirect hits if, as is likely, the radius is over 200 yd. 
The FXR had to be “depressed” below the bottom 
of the wake so that its sound would not be muffled in 
any direction. There was, however, no harm in its 
towing somewhat to one side of the wake while still 
maintaining its depth. 

Consideration was given to a noisemaker fixed 
with respect to the ship but on some bearing other 
than astern. These other positions, which might have 
been accomplished by concentrated machine gun 
fire, seemed to be inferior. 14ie chance of direct hits 
increased since the ship cut across more tractrices. 


DEVELOPMENT OF TOWED NOISEMAKERS 


171 



Figure 8. Typical trajectories relative to a 16- to 18-kt 
ship with two towed noisemakers. Similar trajectories 
from starboard side. 


For safety from indirect hits it was necessary to keep 
the NM within 300 yd of the screws. Most important, 
the ship was in a better position to capture the tor- 
pedo as it circled back from the NM. Finally, the 
sonar interference would be increased. 

Trajectory analysis confirmed the conclusion that 
two NM’s towed abreast gave a more certain counter- 
measure than a single FXR. The advantage of the 
British double FOXER lay in its elimination of prac- 
tically all the ambiguity with regard to stern chases. 
Even a torpedo with zero sensitivity abaft the beam 
(e.g., the cos 0 pattern in Figure 5) could be relied 
upon, after it has passed over one NM, to be drawn 
to the other before getting too close to the ship, as 
shown in Figure 8. During the initial approach the 
FOXER system is equivalent to one NM with the 
same total output. The torpedo is either on a tractrix 
toward one of the NM’s or towards the “acoustic 
center of gravity’’ between them. Thus the chance of 
a direct or indirect hit is essentially the same for the 
double FOXER as for a louder single FXR. 

Highest priority was given to the development of 
adequate diverters. However, as the British were 
finding out, the production and handling difficulties 
seemed to be extremely great with FOXER. Since 
most of the evidence indicated the adequacy of a 
single NM, the decision was made by the United 
States to issue FXR. Emphasis was placed, however, 
on the importance of examining each bit of new evi- 
dence to make sure that this decision was not invali- 
dated. 

15.2.4 Detailed Intelligence Concerning 
the German Torpedo 

As the ideas described above were being worked 
out in detail during the fall of 1943, further infor- 


mation was accumulated on the operation of the 
German torpedo. In mid-October a detailed and ap- 
parently accurate description of the T-4 acoustic tor- 
pedo was obtained from a prisoner of war [P/W] 
whose last cruise had started on July 20 with three 
T-4’s aboard. He also reported that an improved 
model, the T-5, was expected for operational use in 
August. The T-4 was a modification of the electric 
G7e torpedo slowed to 20 knots and with larger rud- 
ders to give it a 90-m turning radius. A liquid-filled 
plastic nose enclosed two forward-facing hydro- 
phones. In a pre-installation test (“Spatz’’ test) the 
rudder was seen to have three positions, hard over on 
each side and occasionally central, as a noise source 
was moved about the nose. A switch was to be set on 
either “WS’’ or “NS’’ at the time of firing. This 
switch had an undecipherable effect on the acoustic 
rudder control, which did not become clear until 
much later. 

Another prisoner of war who had observed run- 
ning tests of the torpedo in the Baltic confirmed the 
general features of our trajectories, but added a 
noticeable weave. This tended to confirm the belief 
that flip-flop control was used. 

With the prisoner of war’s description of the T-4 
nose as a starting point, arrangements were made to 
reconstruct the hydrophones in order to obtain their 
sensitivity pattern. By December preliminary results 
indicated that the T-4 had velocity hydrophones 
connected by phasing circuits and sensitive at about 5 
kc. The sensitivity pattern would only be about 25 



Figure 9. Sensitivity pattern of reconstructed T-4 hydro- 
phone system. 




72 


COUNTERMEASURES TO THE GERMAN ACOUSTIC TORPEDO 


db down astern. All this was welcome news, tending 
to confirm the practical pattern, though complete 
measurements were not made until somewhat later. 

All was not rosy, however. Measurements were 
made with equipment which immediately broke 
down permanently; these tentatively indicated that 
the ship’s own sound was not attenuated in its wake 
at frequencies below 10 kc. Thus the wake could not 
be depended upon to turn the torpedo after a stern 
chase. Full reliance had to be placed on the sensitiv- 
ity’s not being more than 30 db down astern.® 

Scattered information was indicating that the T-5 
was in use and that it had a speed of 24i/2 knots. This 
increased the importance of trajectories involving 
low-speed ratios and also increased interest in how 
self noise had been reduced. One of the more fantas- 
tic reports claimed that for acoustic insulation and 
extra explosive power much of the torpedo had been 
stuffed with guncotton! There seemed to be two 
models of T-5, one with a round plastic nose like the 
T-4 and the other with a flat nose. Imbedded in this 
flat metallic surface were four parallel laminated bars 
10 by 21/2 cm, their long dimensions vertical. These 
were thought to be the surfaces of four magnetostric- 
tion hydrophones. It seemed likely, however, that 
there was a phasing system between the hydrophones. 
4 heir spacing indicated a peak response at about 
121/2 kc. 

There was definite evidence of a magnetic attach- 
ment to the T-4’s pendulum-type inertia pistol. Sev- 
eral ships had reported explosions in their wakes 
which were ascribed to malfunctioning of the inertia 
pistol. Prisoners of war claimed that this had been 
corrected. The magnetic feature increased slightly 
the chance of a direct hit and, more important, al- 
lowed the depth setting to be as much as 10 ft below 
the keel. This lower depth should have considerably 
decreased both the chance of broaching and the self 

o The British even proposed a complicated hypothetical rud- 
der control for T-5 which woidd he able to make a stern chase 
hit over one NM despite the 30-dl) restriction. The phasing sys- 
tem between the hydrophones was sufliciently complicated to l)e 
able to distinguish between loss of contact when the torpedo 
passes over a noise source and loss of contact when the torpedo 
passes beside the source. In the first case the rudder would 
straighten until differential was again obtained, .so that having 
made a stern chase on an NM the torpedo continues toward the 
ship without circling. In the second case the rudder locked over 
in the direction of the source, thus assuring more than one pass 
at a ship by a shot from ahead. This proposal seemed too com- 
plicated to he practical. In addition, the slight weave of the 
torpedo on a stern chase would often cause it to pass somewhat 
to one side of the NM, possibly causing the rudder to lock over. 


noise. In addition, explosions beneath the ship’s keel 
would be much more damaging than at the side of 
the hull. Thought was given to creating a magnetic 
field about the FXR to detonate the torpedo. 

In late January a complete experimental sensitiv- 
ity pattern was obtained from the reconstructed T-4 
nose. It seemed likely that the round-nosed T-5 pos- 
sessed this same pattern. It conformed to the assumed 
practical pattern in that each channel had its maxi- 
mum sensitivity 25 degrees on its own side of the bow 
and its sensitivity at no angle dropped more than 28 
db below this maximum. However, each channel had 
a prominent secondary lobe at about 60 degrees on 
the opposite bow, which even with the best adjust- 
ment could not be brought more than 2 db below the 
main lobe of the other channel. At all bearings 
greater than 60 degrees the patterns for the two chan- 
nels remained within 7 db of each other. They 
crossed over each other in a quite random fashion, as 
shown in Figure 9. The differential D required for 
steering would have to be 7 db to prevent false or 
confused steering, e.g., turning to port when the 
target was about 110 degrees off the starboard bow. 
Nevertheless it was thought possible that in order to 
get a greater homing range the enemy might have 
chosen D = S db, counting on the low sensitivity 
astern to prevent confusion by hiding the ship’s noise 
in the background. Whatever the D, only a source 
between 5 degrees and 60 degrees off the bow could 
be certain to flip the rudder in the correct direction. 

This pattern improved the stern chase situation 
but increased the danger of an indirect hit. The 
lower fore-aft discrimination and the likelihood of a 
greater D made it more difficult than with the prac- 
tical pattern for the ship to take over the torpedo 
after a stern chase. On the other hand, a louder NM 
was required to prevent the ship from attracting a 
torpedo out of a nearby tractrix. If this torpedo 
circled far enough for the NM to be more than 60 
degrees off its bow, it would continue circling no 
matter how poorly it received the ship’s sound. Usu- 
ally when it got headed toward the screws it would 
be in a position to go in for an indirect hit. A direc- 
tive was issued to the effect that any U-boat should be 
kept more than 20 degrees off the ship’s bow. By 
forcing the tractrices farther from the ship this pro- 
cedure would eliminate most indirect hits as well as 
all direct ones. 

By January 1944 FXR Mk 2 (suitable for speeds 
from 12 to 19 knots) was getting into general use 


DEVELOPMENT OF TOWED NOISEMAKERS 


173 


VE DAY 



Figure 10. Number of hits by German acoustic torpe- 
does, by months (escort vessels, solid; merchant ships, 
crossha tched). 

aboard the larger escort vessels. For lower speeds 
FXR Mk 3 was being issued to smaller craft. The 
chief operational difficulties with FXR Mk 2 arose 
from the limits imposed upon the antisubmarine ves- 
sel’s speed and from the depressor (employed to keep 
the NM below the wake). There were also conflicting 
reports as to whether the production models had the 
specified 12- to 15-db excess in output over the ship. 
A safety factor of several decibels was certainly desir- 
able. With much trial and error a 30-inch parallel 
pipe NM was developed which could be towed at 
any speed from 8 to 25 knots. It was depressed by a 
90-lb weight called a “minnow.” Its output was 
steadier than that of FXR Mk 2 and was, at 5 kc, 20 to 
25 db above the average destroyer escort [DE]; this 
compared favorably with the Canadian CAT gear 
and the components of the British FOXER. 

Meanwhile, care was taken to investigate all tor- 
pedoings in an effort to determine the part played by 
T-5’s. Figure 10 records for each month the number 
of incidents which were designated as probable T-5 
hits on the basis of intelligence available before the 
German surrender. (Complete German records are 
still not available.) The importance of towing NM’s 
in dangerous regions was emphasized by the high pro- 
portion of escort incidents (7 out of 14 through May 
1944) in which the presence of the U-boat was unsus- 
pected until the range was less than 2500 yd. In these 
cases no other countermeasure could have been effec- 
tive. The sudden burst of antisubmarine ships hit in 
May 1944 included one case with FXR Mk 2 being 
towed. The newly developed gear was soon issued 
and was designated FXR Mk 4. 

In the spring of 1944 a copy of a T-5 firing table 
was recovered from a U-boat scuttled off India. By 
substitution into firing triangle relationships the tor- 
pedo’s speed was confirmed to be 24 1/^ knots, at least 
on the initial gyro run. Investigation of the mini- 


mum firing ranges (tabulated as a function of the 
target’s firing course and speed) revealed that the 
torpedo had a 500-m enabling run, that is, it was set 
so that the hydrophones could not control the steer- 
ing until 30 sec after firing. This was to help prevent 
the torpedo from homing on the U-boat. The mini- 
mum firing range was specified so that after this 500 
m the screws would still be at least 200 m ahead; then 
there was no chance of the forward-looking lobes not 
hearing the ship. 

Although information concerning the torpedo was 
thus growing steadily, it was still rather fragmentary 
and unsatisfactory in the spring of 1944. This was 
soon to be changed. 

Studies of Captured T-5’s 

On June 5, 1944, USS Guadalcanal and escorts cap- 
tured U-505 with two round-nosed T-5’s aboard. The 
first examination of these torpedoes showed that in- 
side each plastic nose there were not only two hydro- 
phones and some liquid but also a rubber horn struc- 
ture directing each hydrophone 30 degrees off the 
bow. Each hydrophone governed a channel without 
phasing. The maximum sensitivity of the whole sys- 
tem was at about 27i/2 kc, considerably higher than 
had been thought. This higher frequency was all for 
the good since NM output had been found to drop 
off less rapidly with increasing frequency than did 
ship noise. Thus FXR Mk 4 was about 30 db over a 
DE at 27 1/2 kc. 



Figure 11. Measured sensitivity pattern lor captured 
round-nosed T-5, from U-505. 




174 


COUNTERMEASURES TO THE GERMAN ACOUSTIC TORPEDO 


By August measurements of the sensitivity pattern 
revealed a more unpleasant surprise. The response 
astern was 42 db below the maximum! It followed 
that there was some chance of stern hits with FXR 
Mk 4, especially since D, the differential needed to 
flip the rudder, seemed to be only 2 db. On the other 
hand, the patterns for the two channels, shown in 
Figure 11, were well separated (by about 12 db) be- 
tween the angles of 20 degrees and 125 degrees off the 
torpedo’s bow. This together with the low D and 
high NM excess eliminated the danger of indirect 
hits. In fact there were no indirect hit trajectories 
even with an NM 400 yd astern. Since the longer tow 
gave better protection from stern chases, adding more 
cable would have been recommended, had actual 
running tests with the T-5’s not been expected very 
soon. 

Such tests were particularly necessary because of a 
new complication which was discovered. The tor- 
pedo was found to have a common amplifier into 
which the two channels were switched alternately 108 
times a second. If the signals were equal there would 
be no 108-cycle component in the output. Otherwise 
the phase of this component indicated which signal 
predominated. Trouble arose from the fact that FXR 
Mk 4 pipes struck together with a frequency from 90 
to 115 cycles. What was the torpedo response to a 
signal with this modulation? The answer was com- 
plicated by the effect of the automatic volume con- 
trol [AVC]. 

Some questions were, however, definitely answered 
by inspection of the torpedoes. The meaning of the 
“WS” and “NS” settings had been pretty well de- 
duced from prisoner-of-war information. Now there 
was confirmation. WS was set on a torpedo being 
fired from within 90 degrees of the target’s bow; this 
required that when differential was lost the torpedo 
would continue in its minimum turning circle— that 
is, flip-flop control. NS was used on rear shots and 
made the torpedo return to its original gyro course 
after receiving no differential for several seconds; 
when differential D was again received the rudder 
would again go over. 

The reason for having both settings seems to lie in 
the fear that the torpedo, as it approaches a ship from 
a distance, might start circling as a result of a tem- 
porary increase in noise level but before it could 
receive a steady signal from the ship. This was ac- 
ceptable on forward shots since the ship’s motion 
was likely to close the range to where homing could 


start and since flip-flop control was needed if the tor- 
pedo should miss the stern on its first pass. On stern 
shots the flip-flop was not needed because the torpedo 
could stay on its tractrix. Resuming gyro control after 
a temporary signal was likely to bring it into homing 
range of the retreating ship, whereas circling would 
leave it hopelessly behind. 

A consequence of this control system was that even 
if WS shots were successfully prevented from making 
stern chase hits, there might be danger of such hits by 
torpedoes fired on NS from within 30 degrees of the 
ship’s stern. These NS trajectories on passing over the 
NM into a region of no differential would take up a 
gyro course which would take them towards the ship 
to a position from which the screws could guide them 
in to hit. An NS torpedo fired from farther off the 
stern would return to a gyro course from which it 
could never hear the ship or, when it did, it would 
turn far enough to stern to be recaptured by NM. 

Preliminary running tests with the torpedoes re- 
vealed a turning radius of 80 yd and a speed of 22 
knots on turns and confirmed 24p2 knots as the speed 
on gyro control. 

During October, one year after the first intensive 
countermeasure studies, detailed observations were 
made on 20 acoustic runs with the T-5’s apparently 
in proper operating condition. FXR Mk 4 provided 
good protection. FXR Mk 2 proved equally effective 
except on one run during which its output was er- 
ratic. The explanation of this good news lay in the 
very large weave with which the torpedoes ap- 
proached these NM’s. The torpedo’s beam was often 
presented to the NM before the rudder flipped to 
carry the missile into the next phase of the weave. 
When the ship speed was 15 knots or over, the tor- 
pedo was not able to overtake the NM but weaved 
back and forth behind it. With a slower target the 
T-5 was able to circle in ahead of the NM occasion- 
ally, but always at such an angle that the NM con- 
tinued to control it, at least until it had fallen behind 
the NM again. There were no passes directly over the 
NM. Most runs were on NS with the gyro course 
parallel to the ship in order to have conditions most 
favorable for a stern chase hit. Even this did not cause 
trouble. The ship’s wake was so narrow compared 
with the weave amplitude that it had no effect. No 
feature of the trajectories could be correlated with 
the NM’s striking frequency, although in one case 
there were less than 2 cycles between the striking and 
the torpedo-switching frequencies. 





DEVELOPMENT OF TOWED NOISEMAKERS 


175 


The large weave increased the angle off the target’s 
bow from which a torpedo might be fired and still 
make a direct hit. A torpedo fired 1 1 degrees off the 
bow made such a hit, but three shots from about 20 
degrees missed by 100 ft or more. It seemed that the 
existing doctrine (a precaution against the now non- 
existent indirect hits) of keeping the U-boat more 
than 20 degrees off the ship’s bow allowed a sufficient 
margin of safety against these direct hits. A statistical 
analysis of the weaving paths verified the rule of 20 
degrees. Because the weave amplitude increased as 
the torpedo approached the NM, it was found that 
shortening the FXR towing distance from 200 yd 
would markedly increase the danger of direct hits. 
On the other hand, using a 400-yd tow would only 
slightly improve matters. 

Three runs were made against a 16-knot DE (33- 
db spectrum level above 0.0002 dyne per sq cm at 
200 yd) without an NM. The weave on stern ap- 
proaches was quite small, the ship seldom getting 
more than 20 degrees off the torpedo’s bow. This was 
enough, however, to reduce the speed made good to 
about 191/2 knots, so that a ship making 20 knots or 
more should be safe from all but shots down the 
throat. 

On one of these runs the torpedo turned toward 
the ship leaving gyro control at a range of about 1500 
yd. Whether or not this was a fluke, the homing range 
was definitely over 600 yd, since the torpedo, on being 
enabled at this range, steered directly for the ship. A 
1500-yd homing range would make tactical counter- 
measures at speeds of 10-18 knots (step-aside proce- 
dures) extremely risky. 

On the basis of the performance of FXR Mk 2 in 
the running tests, development was initiated on an 
NM with a steady output about that of a good FXR 
Mk 2, i.e., about 20 db over the ship. This gear was 
to be substituted for Mk 4 whenever the latter’s in- 
terference with sonar was a serious handicap. How- 
ever, no satisfactory design was found.p 

During the fall the electronic parts of several flat- 


P Sonar interference from Mk 4 was at times so troublesome 
that a device was developed to turn it off completely when de- 
sired, designated as Harp. The noise output of Harp can be 
turned off or on by quick slackening of the towing cable. It 
should be kept in mind, however, that stopping the NM exposes 
the ship not only to torpedoes fired during the quiet period but 
also to torpedoes fired within 10 minutes prior to its stopping, 
which may have been pursuing the NM. Most WS torpedoes 
which had been trapped by the NM would then hear and attack 
the ship. 


nosed T-5’s were found in France and sent to Eng- 
land. As had been predicted the hydrophones were 
combined by phasing circuits into two channels. The 
maximum sensitivity was at 27 1/2 kc. The amplifiers 
were of a different type from those found in U-505’s 
round-nosed torpedoes. It was established, however, 
that either the flat-nose or the round-nose hydrophone 
could be used with either amplifier. Since the Ger- 
man firing instructions made no distinction between 
models, it seemed likely that there should be little 
difference in their performance. With this informa- 
tion and encouraged by United States running tests 
the British in December authorized the use of Uni- 
FOXER, a single-NM system.^i However, since no ex- 
planation of the large weave was available and since 
it was thought that the large weave might not occur 
with the newly discovered amplifier, the Uni- 
FOXER was towed at 400 yd. 

The large weave was not explained until after the 
war, when an extensive study of the round-nose T-5 
electronic system was completed. The differential D 
required to flip the rudder was found to depend on 
the type of noise source and upon its intensity. D was 
only 2-3 db with ship noise, which was essentially 
thermal noise with a peak to rms ratio of about 14 db. 
D was considerably greater for parallel pipe noise- 
makers, however, whose peak to rms ratio was about 
20 db (in deep water). This D rose to over 12 db when 
the hydrophone output was in the high-voltage range 
corresponding to a signal 55 to 85 db above 0.0002 
dynes per sq cm from the direction of maximum sen- 
sitivity. This corresponds to ranges of 800 to 30 yd 
from a 15-knot FXR Mk 4. Since at no bearing were 
the sensitivity patterns of the two channels more than 
12 db apart, the rudder could not respond when the 
signal was so loud. Thus the torpedo kept turning 
until the NM was so far off its bow that the hydro- 
phone output was reduced to where steering differen- 
tial was obtained. This might not happen until the 
NM was on the torpedo’s beam. Thus the large weave 
resulted. The response was found to be more erratic 
when the NM striking frequency was close to the 
switching frequency. The AVC, the switching cir- 
cuit, and the assorted time constants all contributed 
in such a complicated way that further analysis here 
is not warranted. 


q The British double Foxer always had serious practical diffi- 
culties, but a simple light diverter (Scate) had been developed 
in the United States by the end of World War II which should 
make a two-NM scheme feasible. 


176 


COUNTERMEASURES TO THE GERMAN ACOUSTIC TORPEDO 


Experiments with FXR noise equal in both chan- 
nels and with ship noise superimposed in one (simu- 
lating the critical conditions after passing an NM on 
a stern chase) showed that the ship could flip the rud- 
der when its rms contribution was about equal to the 
rms FXR level in both channels. This meant that 
without the weave a of 3 db could be counted on in 
the stern chase; the earlier calculations based on a D 
of 1-2 db were pessimistic. Study of the circuit dia- 
gram of the flat-nosed T-5 amplifier suggested that, 
even if it did not give a weave, it quite possibly would 
respond to peaks of FXR signal rather than to the 
rms value. FXR Mk 4 should then have a good chance 
of drowning out the ship on a stern chase. 

With present information, were it not for the large 
weave a quiet NM such as Mk 5 could not be advo- 
cated, and towing FXR Mk 4 at 400 rather than 200 
yd would provide a desirable margin of safety. Had 
World War II continued, running tests with the flat- 
nose torpedoes would have been very much worth- 
while, but the German surrender was in itself a com- 
pletely adequate countermeasure. 

15 3 OPERATIONAL DATA 

The success of our countermeasures could not be 
judged accurately from our own operational data, 
since they are largely restricted to cases in which the 
torpedo hit. We have no record of most of the fail- 
ures. Nevertheless certain data are of interest. By V-E 
Day 51 torpedoings had been designated as probable 
T-5 hits. Figure 10 gives the data on a monthly basis. 
As the U-boat effort declined, the low but constant 
contribution of T-5’s became more important. In 
the last months of the war serious consideration was 
given to the problems of issuing NM gear to mer- 
chant vessels. Table 1 summarizes the available in- 
formation on incidents thought to involve T-5’s. 

Of the seven ships hit while towing NM’s, one had 
FXR Mk 2, one FXR Mk 4, two British Uni-FOXER 
(in poor condition), and three had Canadian CAT. 


At least two of these cases were most likely to have 
been direct hits. 

The T-5’s which made hits on antisubmarine ships 
were not so successful as other torpedoes in sinking 
their targets. This may probably be associated with 
the high proportion of T-5’s which struck near the 
stern (63 per cent for antisubmarine ships, 53 per 
cent for merchant vessels), since this often resulted in 
only local damage which could be kept under control. 

It is hoped that a much clearer picture of NM 
effectiveness will be obtained from study of German 
Admiralty IBM cards. These cards give facts perti- 
nent to each torpedo expended (by U-boats which 
got home to report). Interrogations have yielded a 
wide variety of estimates of the number of T-5’s fired 
and their success. It is clear, however, that only a 
small percentage of the torpedoes expended hit their 
targets. There were probably many duds, as is likely 
with such a new and complicated device. The enemy 
must have attributed some failures to our noise- 
makers. It has been reported that orders were issued 
that when an NM was near, T-5’s should not be fired 
unless the U-boat was almost in the target’s wake. If 
this order was followed, the NM’s served their pur- 
pose in a very simple manner. 


Table 1. Use of German acoustic torpedo T-5. 
(September 9, 1943 -May 8, 1945.) 


Type of target 

Antisubmarine 

Merchant 


ship 

vessel 

Numljer of probable T-5 hits: 



With a towed NM 

7 

0 

With target speed under 9 kt 

3 

1 

Probably without countermeasures 

22 

18 

Total 

32 

19 

Percentage of all incidents that were 



probable T-5 hits 

40 

7 

Percentage of T-5 hits causing sinking 
Percentage of other hits causing 

44 

79 

sinking 

84 

82 


EPILOGUE 


F oregoing chapters have discussed the antisubma- 
rine aspects of World War II in some detail, both 
as a history and as an object lesson in rational naval 
tactics. This would not have been done if it were not 
feared that a future war might at some time present 
similar problems. Yet the nature of any hypothetical 
future submarine and antisubmarine operations is 
now so uncertain that any discussion of them is 
highly speculative in character. 

It is evident that this volume on antisubmarine 
warfare has been essentially historical in nature. It 
has retailed facts and figures from experience col- 
lected in the 6 years prior to V-J Day and developed 
theories to explain and interpret them. Its basis is 
therefore a dual one, the characteristics and tactics of 
the German U-boat on the one hand, and those of 
Allied antisubmarine craft on the other. Had the con- 
testants been different ones, the course of the war 
would have been altered correspondingly. 

The outstanding characteristics typical of the Ger- 
man U-boat throughout most of AV^orld War II were 
related to the policy of surfaced operation. Their of- 
fensive tactics were predicated on the use of visual 
detection and tracking on the surface with high speed 
and maneuverability. Diving was resorted to only in 
emergency to escape detection or attack. The large 
wolf packs formed against North Atlantic convoys 
were characteristic of their emphasis on coordination. 
Their consequent heavy radio traffic provided im- 
portant information to the Allies, and their weakness 
in radar detection techniques gave the Allies a telling 
advantage against the surfaced U-boat. 

On the Allies’ side, the overriding importance of 
maintaining North Atlantic convoys to Britain did 
more than anything else to determine the general 
course of the antisubmarine war. This was the central 
battle, with a great variety of diversionary forays and 
skirmishes spread over the remainder of the oceans. 
To defeat the U-boats, their weak points were ex- 
ploited to the full, especially by radio direction find- 
ing position estimates and effective use of radar, both 
surface and airborne. 

The picture would no doubt be different in any 
future war, for many important changes were in 
progress during the closing period of World War II. 
These trends are the most obvious indication of what 
may be expected in the future. 

After the defeat of the U-boats in the summer of 


1943, the Germans initiated an extensive program of 
research and development on methods of U-boat war- 
fare. The high-submerged-speed submarine was prob- 
ably the most striking result. The ultimate objective 
was a submarine with turbine propulsion, burning 
fuel oil with hydrogen peroxide as an oxygen source 
(the Walter turbine). This was to be the Type XXVI 
U-boat with a maximum submerged speed of 25 knots 
for 6 hours instead of 8 knots for 1 hour like the stand- 
ard type of U-boat. The tactical value of such a speed 
in attacking convoys and in avoiding search and 
counterattack would obviously be very considerable. 
Although the feasibility of Walter turbine propul- 
sion had already been demonstrated at that time, no 
Type XXVI U-boats were ever built because of pro- 
duction difficulties.^ Several smaller boats. Type 
XVII, were built, however, for experimental pur- 
poses, and trials were in progress by V-E Day. 

Less spectacular than the turbine drive U-boat was 
the high-speed electric drive Type XXI. They were 
highly streamlined boats with powerful electric mo- 
tors and high-capacity batteries. The resulting capa- 
bilities were a top speed of 15—18 knots submerged for 
a brief period, and of 10 knots submerged for about 
10 hours. U-boat construction was concentrated on 
the Type XXI during 1944 and considerable num- 
bers of them were ready to start operations in May 
1945. No war patrols had actually been made, how- 
ever. 

Both of these types were, of course, to be fitted with 
Schnorchel, which should also be classified as a post- 
1943 innovation. The idea was not a new one, but its 
widespread introduction in the summer of 1944 dras- 
tically changed the complexion of the antisubrnarine 
war, as was pointed out in previous discussion. 
Schnorchel must certainly be reckoned with in 
estimating future trends, and future Schnorchels, 
equipped with radar camouflage, may be expected to 
be even more effective than those which the Germans 
used. 

Significant changes were being made at the end of 
World War II not only in submarine design, but also 
in weapons for submarine use. The acoustic torpedo 
discussed in Chapter 15 is the most familiar example. 


a The hydrogen peroxide required was expensive to produce 
and the majority of the supply was used by the German Air 
Force, especially in the V-l)dmb program. 




177 


178 


EPILOGUE 


but there were other developments as well. Long- 
range and zigzag torpedoes were introduced for use 
against convoys, considerably increasing the proba- 
bility of hit. Fortunately few U-boats had opportu- 
nities to fire them. The Ingolene torpedo, with the 
Walter turbine propulsion giving long range and 
high speed, was developed but not put into opera- 
tional use. Such weapons may be expected to increase 
the potential effectiveness of submarines in the 
future. 

On the Allied side end-of-war developments were 
mostly of the nature of improvements to existing craft 
and weapons, since they were operating with good 
success. New types of sonar for improved detection 
and attack were under consideration, in particular 
scanning sonar which gave an instantaneous plan 
position indicator plot of target position. More effec- 
tive attack weapons were under development, and 
the recently introduced Squid gave evidence of hav- 
ing a probability of success in attacks about ten times 
that of ordinary depth charges. 

Certain more general developments will also un- 
doubtedly have profound effects on future antisub- 
marine warfare, just as sonar, radar, and the aircraft 
profoundly affected it during World War II. Atomic 
explosives and power utilizing nuclear energy come 
immediately to mind as the most revolutionary of re- 
cent introductions. It is impossible to estimate the 
effects of such developments now; all that can be 
done is to point out that they are likely to be consider- 
able. Somewhat less striking, but also of great im- 
portance, are the very extensive developments of 
guided missiles. Homing torpedoes may be con- 
sidered as a particular class of weapons of this type. 

What, then, are we to conclude that the future of 
submarine and antisubmarine operations will be 
like. Some conclusion is in order even though we rec- 
ognize that it can only be a wildly speculative one. 

The whole state of naval warfare in the future is 
uncertain, but it can surely be agreed that control of 
the sea, including the depths beneath the surface and 
the space above it, is of prime military importance, 
and such control may rightly be considered as the ob- 
jective of naval power. How such control may best be 
accomplished is a question for future analysis and 
planning to decide. The general means available are 
naval craft and missiles; for modern warfare, no 
longer a matter of personal combat, is based on the 
missile, the means of implanting a destructive agent 
in the enemy’s midst from long range. Naval craft- 


ships, aircraft, and submarines— are fundamentally 
missile-carriers whose aim is to launch missiles so that 
they reach the proper place. 

The characteristics of each type of craft are deter- 
mined in part by the requirements of the missile 
which it launches. In part, however, they are intrinsic 
—speed, endurance, and maneuverability. Relative 
ease of detecting the enemy and being detected by 
him are also of great importance. 

In the past submarines have been built around the 
torpedo as missile. Improvements in torpedo design 
and the possible introduction of submarine-launched 
guided missiles may significantly alter its role in the 
future. The great intrinsic advantage of the sub- 
marine is its invisibility, in which it still exceeds all 
other types of craft. Means for overcoming this invisi- 
bility are likely to remain the chief concern of anti- 
submarine measures. Their detailed nature must, 
however, be determined in terms of the use to which 
submarines are put and of the type of submarines 
involved. 

A satisfactory estimate of the most probable enemy 
use of submarines can hardly be made without first 
deciding on the general tactical and strategical situa- 
tion which is likely to exist. We should first deter- 
mine who will be fighting whom, what bases and 
facilities each will have, what supply lines or other 
objectives are open to enemy submarine attack. Set- 
ting up such a complete problem is, however, beyond 
the scope of this discussion, but we can still make 
some predictions in general terms. 

In the first place it is reasonable to expect that sub- 
marine tactics which proved to be highly effective in 
World War II will be tried again in the future. In 
particular the use of submarines to attack merchant 
shipping is likely to be repeated by our enemies so 
long as our strategy is dependent on such ships. In 
any major war this is likely to be the case, since we 
will use ships to supply bases outside the continental 
United States and to import necessary materials of 
war. Only if the enemy expected to win such a rapid 
and crushing victory as to make destruction of our 
shipping unnecessary would he be expected to ignore 
the importance of a submarine campaign against 
merchant ships. 

The developments of the last period of World War 
II support this point of view. The new types of Ger- 
man U-boats which were designed and constructed 
after 1943 were intended for the same basic purpose 
as the earlier ones— to attack merchant ships. No 


C« 


:L 


EPILOGUE 


179 


major change was envisaged, but it was hoped that 
the increased U-boat speed would restore their tacti- 
cal advantage and permit them to resume effective 
attacks against convoys, even when operating sub- 
merged to gain safety from aircraft. 

The use of submarines for anti-shipping opera- 
tions involves use of the torpedo as the primary 
weapon. New developments suggest, however, that 
different weapons, such as a guided missile of the 
V-bomb type, might also be launched from subma- 
rines. If such a missile were designed to carry nuclear 
explosives, the destructive power of the weapons 
which even a relatively small submarine could carry 
would be many times greater than that of a battleship 
or carrier at present. Weapons might well be launched 
with accuracy comparable to that of present gunnery 
or bombing. Since the submarine is an all-but- 


invisible craft, developments of this sort might be 
expected to be extremely effective. 

The detailed evaluation of the effectiveness of pos- 
sible types of future submarine operations is beyond 
the scope of this discussion and could not be made 
now in any case. It is first necessary to determine the 
fundamental capabilities of the craft and missiles 
that may be involved, with all possible new develop- 
ments and improvements. When this has been done 
the tactical evaluation can be undertaken. 

It does appear, however, that future developments 
are not likely to eliminate the submarine’s great 
merit, its relative invisibility. At the same time the 
striking power of submarines is likely to increase. We 
may safely conclude that submarine and antisub- 
marine warfare will be highly important phases of 
any future naval conflict. 




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APPENDIX I 


AGREEMENT BE 1 \\ EEN 
ALLIED ASSESSMENTS OE ATTACKS ON 
U-BOAl S AND THE RESUL LS REVEALED 
AI SURRENDER 

A t the end of AVorld W^ar I it was found that the 
- number of U-boats which had actually been 
sunk was rather less than the number presumed stink 
according to the assessment list. At the start of World 
War II it was intended that the assessment policy be 
a little more realistic, and more convincing evidence 
was demanded to secure an assessment of “sunk” (A) 
or “probably sunk” (B). 

The Italian surrender was followed by the publica- 
tion of a list of the Italian U-boats which did not re- 
turn to base prior to the armistice. A\dren compared 
with the Allied assessments for attacks thought to 
have been made on Italian U-boats the agreement 
was excellent. There were 72 A and 7 B assessments, 
a total of 79 Italian U-boats sunk or probably sunk, 
whereas the Taranto list showed 80 submarines to be 
missing. Furthermore intelligence had provided the 
names of 67 of them before the armistice, and these 
names all checked. 

The German surrender provided a list of the 
U-boats lost with the name of the commander and 
date and position of the sinking. A comparison of the 
A and B assessments and the losses shown in the Ger- 
man list follows: 

Allied assessments compared with German list* 

German 




A 

B 

Total 

list 

I 

Sept. 39 -June 40 

24 

0 

24 

24 

II 

July 40- Mar. 41 

13 

7 

20 

13 

III 

Apr. 41 -Dec. 41 

26 

1 

27 

27 

IV 

Jan. 42 -Sept. 42 

27 

23 

50 

50 

V 

Oct. 42-Iune 43 

79 

49 

128 

144 

VI 

July 43 -May 44 

117 

79 

196 

206 

VII 

June 44 -May 45 

105 

54 

159 

179 


Sept. 39- May 45 

391 

213 

604 

643 


* These data are based on information available at V-E Day. Neither 
Allied nor German information is complete for the last periods. Hence 
the figures given here do not agree in detail with those presented in 
Chapter 8, based on more complete records available several months 
later. Nevertheless the agreement of Allied and German estimates 
proves the overall accuracy of the assessments made during World 
War H. 

It is clear that the Germans lost more U-boats as a 
result of Allied action at sea than the combination of 


A and B assessments would indicate. This is to be ex- 
pected as losses due to mines, or perhaps ordinary 
hazards of the sea, would not be known to the Allies. 
In only one of the periods did the total A and B assess- 
ments exceed the losses given in the German list. For 
this period, July 1940- March 1941, there were 7 B 
assessments which have never been confirmed as sink- 
ings though intelligence has completed the story of 
the sinkings for this particular period. The percent- 
age of B assessments which actually corresponded to 
sinkings is problematical, but those cases which do 
not represent sinkings are compensated for by lower 
assessments which acttially represented sinkings 
though they were not credited as such. 

A survey of the attacks on Japanese U-boats shows 
38 assessments of A and 62 B assessments, a total of 
100 A and B. The individual losses from Japanese 
lists add up to 123, of which two were from mines and 
two by running aground. The agreement is not as 
good as with the Italian and German lists, but it 
is still satisfactory, particularly when the greater 
difficulty of obtaining intelligence is considered. 

The A and B assessments for the losses by the three 
Axis powers are given below and a significant differ- 
ence is obvious. 



A 

B 

Total 

Enemy Loss 
Lists 

Italian 

72 

7 

79 

80 

German 

391 

213 

604 

643 

Japanese 

38 

62 

100 

123 


501 

283 

783' 

846 


1 he ratio of A to B assessments is very high for 
Italians, intermediate for Germans, and low for the 
Japanese. This relationship would seem to be due to 
at least three main factors: 

1. The Italians gave up easily, surfaced, and sur- 
rendered, thus giving sure proof of destruction, 
whereas the Japanese seldom surfaced when the game 
was up. 

2. Intelligence information was easier to obtain 
from European sources than from Japanese. 

3. Antisubmarine forces in the Pacihe had less op- 
portunity to remain in the vicinity to search for evi- 
dence of destruction because of other fleet duties. 

The complete picture for \Vorld War II allows the 
conclusion that the summation of A and B assess- 
ments for attacks on Axis U-boats gives a total which 


181 




APPENDIX 


is close to that of the actual losses and useful for prac- 
tical purposes. Attempts to correlate B attacks with 
the loss of individual U-boats were not always suc- 




cessful and showed that the validity of any particular 
B assessment as evidence of the destruction of a 
U-boat is questionable. 





GLOSSARY 


Aces. U-boat commanders of outstanding records, credited 
with sinking large amounts of Allied shipping. 

Acoustic Torpedo. A torpedo ^vhich detects the target ship by 
means of sound and is controlled by this sound so as to steer 
towards the ship and eventually hit it. 

Admiralty. The headquarters staff of the British Navy. Also 
used in the same sense for German Navy as “German Ad- 
miralty.” 

Ahead -Thrown. (A) Weapon: an antisubmarine weapon used 
by surface craft which is projected ahead of the ship, usually 
to a distance of about 300 yd; (B) attack: an attack employing 
an ahead-thrown weapon. 

Aiming Errors. Errors in aircraft attack arising from inac- 
curacies in dropping bombs, or firing rockets, i.e., errors in 
aiming the weapon. 

A.N.D. Admiralty Net Defense; a net suspended along the 
sides of a ship to catch torpedoes. 

Approach Error. Error in surface craft antisubmarine attack 
which would exist without submarine evasion. See Attack 
Error and Evasion Error. 

Antisubmarine Aitack Plotter [ASAP]. An electronic device 
for plotting positions of own ship and submarine during 
attack. Sonar echoes are plotted automatically on a large 
cathode-ray tube of long persistence. 

Antisubmarine Ship. Any naval ship used for attacking sub- 
marines, i.e., destroyers, destroyer escorts, frigates, sloops, 
corvettes, trawlers, patrol craft [PC], sub chasers [SC], suitably 
equipped minesweepers, etc. 

ASDevLant. Anti-Submarine Development Detachment, At- 
lantic Fleet; an organization set up to test and develop anti- 
submarine ecpiipment and tactics for its use. 

Asdic. British echo-ranging sound gear, equivalent to United 
States sonar. Name is derived from Anti-Submarine Detection 
Internal Committee, which pioneered the development of 
such equipment. 

ASG (Radar). United States Navy airborne S-band (10-cm) 
search radar, introduced early in 1943. 

Assessment. An estimate of the damage done to a U-boat re- 
sulting from an attack by an antisubmarine craft. This esti- 
mate is made by a special committee on the basis of all 
available evidence. Assessments range from “A” — U-boat 
known to be sunk, to “I” — attack not on a U-boat. 

ASV. Air-to-surface-vessel radar, that is, airborne radar for 
searching for surface ship targets. 

AS^V'ORG. Anti-Sulmiarine Warfare Operations Research 
Group. A group of civilian scientists attached to Tenth Fleet 
Headquarters. Later known as ORG when transferred to 
COMINCH Headquarters. 

Attack. Release of one or more weapons against a U-boat in 
a barrage or stick. Several attacks made in succession on the 
same U-boat (i.e., within a few hours of each other) are called 
an incident, but the term attack is sometimes used loosely as 
synonymous with “incident.” 


Attack Error. Distance between mean point of explosion of 
weapons and center of submarine. See Aiming Error, Evasion 
Error, Approach Error. 

Attack Teacher. A mechanical device for simulating the con- 
ditions of a surface craft attack on a submarine, used pri- 
marily for training personnel on shore. 

AVC. Automatic volume control in an electronic amplifier. 

Balanced Force. A force capable of an equally effective of- 
fensive at all times, in particular, both day and night. 

Barrage. A number of depth charges or contact charges re- 
leased as a group in a surface craft attack. 

BDI. Bearing deviation indicator, a modification to standard 
sonar gear giving more accurate bearings by use of lobe 
comparison. 

Biber. a midget submarine with crew of one, developed by the 
Germans and used during 1944-45. 

Blind Time. The time from loss of contact with the submarine 
(or release of w^eapons) until weapons reach U-boat depth. 

Borkum. a German radar search receiver of crystal detector 
type covering the 75- to 300-cm band. 

Braw'Ning Shot. A torpedo shot fired into a convoy without 
being aimed at any particular ship. 

Camouelage (Radar). Any device for reducing the range at 
which an object can be detected by radar, for example, non- 
reflecting coatings for reducing echoes. 

C.\1M Ships. Merchant ships fitted with catapults for launching 
aircraft. 

Catalina [A/C]. Twin-engined seaplane (or amphibian) built 
by Consolidated Aircraft. Long range, bin slow, with mod- 
erate bomb load. 

Class A Attack. An aircraft attack on a submarine in whicli 
bombs are dropped not more than 15 sec after the submarine 
submerges. 

Coastal Command. The branch of the British Royal Air Force 
responsible for ocean patrol and attacks on naval targets. 

Commanded Volume. The locus of points about an exploding 
charge or bomb (or about its trajectory) that have the prop- 
erty that a submarine with center at any of the points will be 
sunk. 

Commodore (of Convoy). A senior naval officer (or occasion- 
ally merchant captain) responsible for the navigation, signal- 
ling, and other such activities of the convoy. The escort com- 
mander, on the other hand, is responsible for its defense 
against the enemy and has ultimate authority. 

Contact. An instance of detection of an enemy unit. For ex- 
ample, when a U-boat sights a convoy it is a contact on a con- 
voy, as when an aircraft receives, a radar blip from a sub- 
marine, it is a contact on a submarine. 

Contact Fuze. A device in a bomb, rocket, or other projectile 
for causing it to explode on hitting the target. 


V 


18.3 


184 


GLOSSARY 


Convoy. A group of merchant ships sailing together, usually 
defended by naval ships acting as escorts. 

Coordinated Attack or Incident. An attack or incident in 
which two or more ships and aircraft (or both ships and air- 
craft) take part. See Attack and Incident. Those involving 
both ships and aircraft are sometimes termed joint attacks (or 
incidents). 

Corvette. A small antisubmarine ship, usually capable only of 
rather slow speed. 

COTCLant. Commander Operational Training Command, 
Atlantic Fleet, in charge of training for the Atlantic Fleet. 

Counterattack. An attack on a submarine which has attacked 
friendly ships or is threatening to do so. 

Countermeasure. Any action or device designed to reduce the 
effectiveness of sonic enemy action or device. Tactical coun- 
termeasures involve changes in tactics or operational proce- 
dures. Material countermeasures involve new equipment. 
These terms are used most frequently with respect to radar 
and homing torpedoes. 

CNO. Chief of Naval Operations— the naval officer (and sub- 
ordinate staff) in charge of the U.S. Navy Department. 

Creeping Attack. A coordinate attack by surface craft aimed 
at surprising the submarine. An “assisting ship” maintains 
sonar contact and directs the “attacking ship,” which pro- 
ceeds at slow quiet speed over the submarine without echo 
ranging and drops charges. 

Cuba Ia. Antenna used with the German Tunis search receiver 
for S-band reception, made up of dipole and parabolic re- 
flector. 

Curly. A torpedo which follows a zigzag course. 

DB. Decibel. A unit of sound intensity. See any textbook of 
general physics or acoustics. 

D-Day. Day of invasion of Normandy — ^June 6, 1944. 

Density. Number of objects per unit area. For example, 
U-boat density might be expressed as the number of U-hoats 
per million square miles of ocean. 

Depth Fuze. A device for detonating a bomb or other weapon 
at a preset depth. 

Destroyer Escort [DE]. A large antisubmarine ship (about 
1,800 tons) of fairly high speed, about 18-24 knots. 

DE. Radio direction finding by intercepting enemy radio trans- 
missions and obtaining their bearings with directional re- 
ceivers, thereby estimating the enemy position. See HEDF. 

Direct Hit (by torpedo). A direct hit is made by an acoustic 
torpedo on a ship using a noisemaker if the torpedo, while 
steering toward the noisemaker, hits the ship. 

Dispersion (of Iiomhs). 4 he variation that would exist iu the 
explosion points of a large numtier of bombs of a given type 
if they were all dropped under the same conditions (in so far 
as conditions could be controlled). 

Diverter. A device for causing a towed noisemaker to tow at 
one side of the ship’s wake rather than directly astern. 

Electrasonne. German code name for a system of radio bea- 
cons used as navigational aids. 


Escort. A naval ship or aircraft used to protect a merchant 
ship or group of shijDs. Antisubmarine ships are sometimes 
termed escorts even when engaged in offensive operations. 

Escort Carrier [CVE], A small aircraft carrier usually built on 
a merchant vessel hull and designed to fly antisubmarine 
planes. Escort carriers were actually used mostly in offensive 
operations, only infrequently on escort duties. 

Escort Commander. Naval officer in command of forces as- 
signed for defense of convoys. 

Escort Group. A group of antisubmarine ships which are 
operated as a unit. 

Evasion Error. Error in antisubmarine attacks introduced by 
evasive maneuvers on the part of the submarine. 

Evasive Routing. Routing to avoid known submarine positions. 

Exchange Rate. Ratio of merchant vessels sunk by submarines 
to submarines sunk by antisubmarine vessels. 

FAT. German designation for a torpedo which runs a zigzag 
course, with nature of a zigzag not adjustable. See LUT. 

Ferret. An aircraft equipped with receivers for monitoring 
enemy radar transmissions. 

Fleet Air Arm. British carrier and ship-based aircraft. 

Fleet Air Wing 7. This air wing was based in Britain until 
the end of World War II. 

Fliege. German code name for S-band antenna of Tunis GSR. 
See Cuba Ia. 

Flip-Flop Control. Rudder control for a homing weapon 
which can be only in central position or hard over port or 
starboard. 

Flying Fortress. A four-engine Boeing bomber used to a small 
extent for antisubmarine patrol. 

Follow-up (of contacts) . Effort to gain contact with a sub- 
marine whose position was accurately known at an earlier 
time by virtue of a previous contact. 

Fourth Fleet. United States naval forces which were based in 
the South Atlantic; a subdivision of the Atlantic Fleet. 

Foxer. British towed noisemaker gear for decoying German 
acoustic torpedoes, consisting of two parallel bar noisemakers 
and diverters. Near the end of the war Uni-Foxer, a single 
noisemaker, was employed. See FXR. 

Frigate. A large British antisubmarine ship (about 2,000 tons) 
of long endurance but moderate speed, 15-18 knots. 

FXR. United States single towed noisemaker of parallel bar 
type (See Foxer). 

Gambit. An aircraft search for follow-up of contacts in which 
the aircraft leaves the most probable initial submarine posi- 
tion and flies at some distance from it for some time, in the 
hope that the submarine will return to the surface where it 
can be sighted when the aircraft returns to the area. 

Gap, Mid-ocean. The mid-ocean area which could not be 
patrolled by land-based aircraft. 

Glider Bomb. German radio-controlled glider bomb which is 
guided to target by parent aircraft. 


GLOSSARY 


185 


Gnat. German Navy acoustic torpedo which homes automati- 
cally on noise from target ship. 

Gross Ton. A measure of ship size based on volume. The ton- 
nage is the entire internal cubic capacity of the ship expressed 
in tons of 100 cubic feet to the ton, except certain spaces which 
are exempted, such as peak and other tanks for water ballast, 
open forecastle bridge and poop, anchor gear, steering gear, 
wheel house, galley, and cabins for passengers. 

GSR. German search receiver, i. e., German radar receiver for 
detecting transmissions by Allied search radar. 

GU. Designation for convoys from Mediterranean and Gibral- 
tar to the United States GUS for slow convoys, GUF for fast. 

Guided Missiles. A missile whose course can be adjusted after 
launching (or firing) so as to hit the target. The adjustment 
may be made by an operator in the launching craft or may be 
automatic. Acoustic torpedoes are examples of the latter type. 

Halifax. A British four-motored heavy bomber built by 
Handley-Page. 

Hedgehog. An ahead-thrown weapon with mortar projected 
barrage of 24 contact charges, trainable to about 20 degrees off 
the botv of the ship. 

HFDF. High-frequency DF {see DF). Frequencies of about 10 
megacycles are involved. 

HG. Designation for convoys from Gibraltar to the United 
Kingdom. 

Hold-down Hunt. A search of sufficient intensity to insure the 
submarine’s being sighted if it surfaces. 

Homing. Guiding to a source of signals. A U-boat in contact 
with a convoy unit’s signals, thereby homing other U-boats to 
the scene. An acoustic torpedo steers towards the sound from 
a ship’s propellers, thereby homing on the target. 

HoS. High resolution radar of S-band frequency for use in 
blind bombing. 

Hudson. Twin-engine Lockheed bomber. 

Hunter-Killer Operations. Offensive operations aimed at 
finding and destroying U-boats, usually involving groups of 
surface craft and sometimes coordinated with aircraft. 

HX. Designation for convoys from Halifax or New York to 
Britain. 

HoX. High resolution radar of X-band frequency for use in 
blind bombing. 

Hydrophone. A receiver of underwater sound. 

Incident. See Att.vck and Coordinated Incident. 

Indirect Hit. A hit made by an acoustic torpedo which is at- 
tracted to the target ship out of a trajectory leading to a 
noisemaker. 

Influence (fuze or pistol). A fuze activated by the change in 
earth’s magnetic field due to the submarine’s presence, which 
is designed to detonate the charge whenever it is close enough 
to the submarine to be effective. 

Intercept Receiver. Radar search receiver {see GSR). 


Jaumann Absorber. A nonreflecting covering which is effective 
against S- and X-band radars, made up of spaced layers of 
semiconducting paper. See WTscii Absorber. 

Lateral Range Curve. A curve which gives the probability of 
detection of an object by a searcher (who is proceeding on a 
straight course) as a function of distance of direct approach. 

Leigh Light. British aircraft searchlight used for night attacks. 

Lethal Area. The area on the ocean surface centered above 
an exploding depth bomb which has the following property: 
In a Class A attack any submarine whose center lies below the 
lethal area may be considered sunk, and no others. 

Lethal Radius. The lethal radius of a given explosive charge 
is the maximum distance from the pressure hull at which it 
may be expected to do lethal damage. 

Liberator. Four-motored heavy bomber built by Consolidated 
Aircraft frequently modified for use in antisubmarine patrol 
because of its long range and large bomb load. 

Life (Lifetime) of a U-Boat. At any time the average life of 
a U-boat is the number of U-boats at sea divided by the rate 
at which U-boats are sunk per month. 

Limiting Approach Lines. The boundary of the region from 
which a submarine can approach a ship or formation, drawn 
relative to the ship or formation. 

Line Error. The component perpendicular to the course of 
attacking aircraft of the distance between center of stick and 
center of submarine. 

Localization. Determination of exact submarine position by 
sonar. 

Lost Contact (Range). Range at which s^pnar contact with 
submarine is lost because submarine is below sonar beam. 

LUT. German designation for a torpedo which runs a preset 
zigzag course. See FAT. 

M.\C Ships. See CAM Ships. 

MAD. Magnetic anomaly detector (or magnetic airborne de- 
tector), an aircraft instrument for detecting distortion of the 
earth’s magnetic field due to the presence of a submarine. 

Maximum Submergence. Submarine tactics tyhich involve re- 
maining submerged as much as possible. Normally about 2 to 
4 hours a day must be spent surfaced or at Schnorkel depth. 

Mean Radial Error. Average distance from center of target to 
center of barrage. 

Metox. Early GSR for long wavelength radar, introduced in 
1942. 

Mk II Radar. British long wavelength search radar used on 
aircraft. 

Mk hi Radar. British airborne S-band search radar. 

Mk 8 Depth Charge. United States influence-fuzed depth 
charge. 

Mosquito. British twin-engined fighter-bomber built by De- 
Haviland. 

Mousetrap. United States ahead-thrown weapon with rocket 
propected barrage of 8 or 16 contact charges. Used in small 
ships. 


186 


GLOSSARY 


MPL Mean point of impact of a group of bombs. 

Mucke. Horn antenna for X-bancl reception with Tunis GSR. 

MV. Merchant vessel. 

Naxos. (1) Early nondirectional GSR effective in S-band; 
(2) Amplifier used in Tunis GSR. 

NM. Noisemaker. 

Noisemaker. An artificial source of underwater noise used to 
conceal the noise made by a ship or submarine or to interfere 
with the operation of, or to decoy, acoustic devices such as 
acoustic torpedoes. 

North Russian Convoys. Convoys from Britain around the 
north of Norway to Murmansk, Archangel, or other North 
Russian ports, and returning convoys. 

OB. Designation used in early years of war for convoys out- 
ward bound from Britain for America and Africa. 

Observant. A search plan used by surface craft, consisting of 
going to point of last contact with submarine and then steam- 
ing around a square centered at that point with 2-mile sides. 

OG. Designation for convoys from Britain to Gibraltar. 

ON. Designation for convoys from Britain to America. During 
latter part of World War II ON was reserved for 9-knot con- 
voys, ONS being used for 7 -knot. 

ORG. Operations Research Group. See ASWORG. 

ORS. Operations Research Section, title of British operations 
research organizations. Most frequently used in ORS/CC 
Operations Research Section, Coastal Command. 

OS. Convoy designation for convoys from Britain southbound 
to Sierra Leone. 

Patrol Craft. Small antisubmarine craft, normally not over 
about 200 feet in length. 

PQ. Designation for convoys from Britain to Archangel. {See 
North Russian Convoys.) 

I’RACTicE Attacks. Simulated attacks carried out either on the 
attack teacher or in exercises with a friendly (“tame”) sub- 
marine. 

Proximity Fuze. A fuze designed to detonate a bomb, charge, 
or projectile, when within a certain distance of the target. 
{See Influence Fuze.) 

P/ W. Prisoner of war. 

Q Attachment. British auxiliary sound gear with beam di- 
rected below the main Asdic beam for maintaining contact 
with deep U-boats at short ranges. 

RGOO. Same as Metox. 

Range Errors. The component parallel to the source of at- 
tacking aircraft of the distance between center of stick and 
center of submarine. 

RDF — Type 286, Type 271. British surface craft search radar. 
Type 286 had fixed aerials and operated on long wavelength. 
Type 271 was S-band gear with rotating beam and PPI pres- 
entation. 


Retro-Bombs. Contact bombs used by aircraft in conjunction 
with MAD. Bombs are propelled backwards by rocket motors 
so as to drop vertically from aircraft. 

Reverberation. The totality of small false echoes received on 
sonar from objects in the ocean, its surface, and the bottom. 

Rocket. Used alone the word implies forward-firing rockets of 
about 3-in. diameter used by aircraft. {See Mousetrap, for 
comparison.) 

Runddipol. Submersible GSR aerial used for long wavelength 
radar reception. 

S-Band. Radar operating band for wavelengths of about 10 cm. 

SBT. Submarine bubble target. A German decoy used to pro- 
duce false echoes from a cloud of bubbles. 

SC. Designation for 7-knot convoys from America to Britain. 

Schnorkel. U-boat combination air intake and Diesel exhaust, 
permitting operation on Diesels when at periscope depth. 
Introduced in 1944. 

SCR-517. United States Army S-band airborne search radar. 
SCR-717. Improved version of SCR-517. 

SD Radar. United States Navy X-band airborne search radar. 

Sea Return. The totality of false radar echoes from ocean 
surface. 

Search Receiver. A radar receiver for intercepting signals 
from enemy search radar and thereby being warned of his 
approach. 

Sensitivity Pattern. The sensitivity of an acoustic torpedo is 
a diagram showing the range and bearing at which a given 
sound source will produce a given signal at the torpedo con- 
trol mechanism. 

SG. United States Navy surface craft S-band search radar {see 
RDF, Type 271). 

Shadowing. A U-boat shadowing a convoy kept in contact with 
it and reported information about it without attacking. 

Sinking Rate. The effectiveness of a U-boat in sinking ships. 
The sinking rate is equal to the U-boat’s sweep rate times the 
fraction of ships contacted that are sunk. 

SL. (1) Convoy designation for convoys from Sierra Leone to 
Britain; (2) radar designation for United States Navy radar 
similar to SG but lighter in weight. 

Sloop. Large antisubmarine ship of moderate speed, similar 
to frigate. 

Sonar. The United States Navy term for underwater sound 
equipment and its use. British use “Asdic” in similar way. 

SoNOBUOY, Expendable Radio. A small buoy capable of trans- 
mitting sounds heard in the ocean to an observer by radio. 
Used primarily by aircraft. 

Sortie. An aircraft flight. 

Squid. An ahead-thrown depth charge and associated depth- 
determining sound gear by means of which the submarine’s 
actual^epth is automatically set into the depth charge fuze. 


GLOSSARY 


187 


Step- Aside. A radical zigzag designed to permit a ship to ap- 
proach a U-boat with miniiniim danger from acoustic tor- 
pedoes. 

Stern Chase. An attack or pursuit course which overtakes the 
target from its stern. 

Stick. A number of bombs or depth charges dropped in a row. 

Straggler. A ship which left a convoy because unable to main- 
tain proper speed. 

Submerged Approach Zone. The area around a convoy from 
which a submarine can make a submerged approach to it. 

Sunderland. British four-motored seaplane. 

Supply U-Boat. A U-boat fitted to refuel and supply other 
U-boats without attacking any ships itself. 

Support Group (i.e., 2nd Support Group). A number of anti- 
submarine ships kept together permanently as a group to 
hunt U-boats and provide extra defense to convoys threatened 
by attack. 

Sweep Rate. The number of contacts made per hour per unit 
of target density by a searching craft. It is expressed in square 
miles per hour and is the measure of searching craft’s effec- 
tiveness of covering area. 

Sweep Width. Sweep rate divided by speed (approximately). 

T-5. German acoustic torpedo (earliest models were designated 
T-4). 

Tenth Fleet. United States Navy Headquarters assigned tasks 
of conducting war on submarines and protecting merchant 
shipping. 

Torpedo Danger Zone. Area around a ship or convoy from 
which a submarine has a good chance of hitting with a salvo 
of torpedoes. 

Torpex. Recently developed explosive superior to TNT in de- 
structive power. 

Toss Bombing (or Rocketing). A method of glide-bombing (or 
rocket-firing) in which weapons are released as the plane pulls 
out of the glide. 

Tracking. Maintaining contact with an enemy unit so as to 
determine its course and speed. 

Tractrix. The curve of pursuit followed by always proceeding 
towards an object which is itself moving with constant course 
and speed. 

Transit, Transit Area. Passage of a submarine from base to 
its operating area is termed a transit and the area between 
base and the operating area is the transit area. 


Tunis. Directional GSR for interception of S-band and X-band 
radar. {See Naxos, Mucke, and Fliege.) 

Type XVII-B U-Boat. Small experimental U-boats with oil- 
HoOo turbine drive for submerged operations having speeds 
up to 24 knots. 

Type XXI U-Boat. New type U-boat with powerful electric 
motors, and extra batteries permitting high submerged speed 
operation (up to 15 knots). 

Type XXIII U-Boat. Similar to Type XXI, but smaller, for 
coastal operations. 

Type XXV^I U-Boat. Proposed ocean-going U-boat with tur- 
bine drive as in Type XVII. 

T YPE 147B Asdic. British sound gear used for determining sub- 
marine depth during an attack. 

Type 271 Radar. British shipborne 10-cm search radar. 

U-Boat. Any enemy submarine of 200 tons or larger. 

UG. Designation for convoys from United States to Gibraltar 
and Mediterranean; UGS for slow convoys, UGF for fast. 

U-Kreuzer. Large long-range U-boat. 

V-1. German “buzz-bomb” or jet-propelled pilotless aircraft of 
moderate range. 

VLR Aircraft. Very long-range aircraft. 

Vixen. An attenuator for use with airborne radar to reduce the 
output as range to the target is closed, thereby confusing the 
GSR operator. 

Wanz G-1. Long wavelength GSR, an improvement on Metox. 

Weapon Lethality. The effectiveness of a weapon in produc- 
ing damage, without regard to the accuracy with which it can 
be placed. 

Wellington A/C. British two-motored bomber and patrol 
plane built by Vickers. 

Wesch Absorber. A rubber-like coating for reducing radar 
echoes. See Jaumann Absorber. 

Whitley. British two-motored bomber and patrol plane. 

Wolf Packs. Groups of U-boats operating together as a unit. 

X-Band. Band of radar frequencies with wavelength approxi- 
mately 3 cm. 

Zigzag. Frequent course changes to make attack by submarines 
more difficult. 


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INDEX 


The subject indexes of all STR volumes are combined in a master index printed in a separate volume. 
For access to the index volume consult the Army or Navy Agency listed on the reverse of the half-title page. 


Acoustic torpedo, German 
countermeasures, 98-99, 161-176 
flip-flop rudder control, 167 
Geier, 75 
T-4; 171-172 
T-5; 56, 172-176 
trajectory analysis, 164-171 
Admiralty net defense, torpedo counter- 
measure, 30, 41, 98-99 
Aircraft as convoy protection against 
U-boats, 39, 109 

Aircraft attack on submarines, 127-138 
attack errors, 128-130 
bombing errors, 137-138 
characteristics of bombs, 133 
depth bombs, 136-137 
effect of altitude on success, 141 
factors determining success, 127-136 
probability of success, 132-136 
use of rockets, 138 
variation of individual missiles, 133 
weapon lethality, 130-132 
Aircraft equipment for use against 
U-boats 

Leigh searchlight, 30 
magnetic airborne detector, 30 
plan position indicator, 29 
radar, 13, 30 

Aircraft patrol, effect on submarine sub- 
mergence, 95-98 
Aircraft tactics against U-boats 
advantages over surface craft, 12 
against Schnorchel-eq nipped U-boats, 
73-74 

depth bombs, 136-137 
depth charges for air use, 12-13 
rocket attacks, 56, 138 
search for surfaced submarines, 139- 
141, 151-152 

sweep widths for visual search, 140 
work of seasearch-attack development 
unit, 29 

Aircraft types, for use against U-boats 
carrier-based aircraft, 55 
civilian air patrol, 29 
merchant aircraft carriers, 36-37 
“scarecrows,” 6 

very long range aircraft, 35-36 
Aircraft warning radar, enemy, 153 
Air-surface vessel (ASV) radar, 14 
AN/ A PS-2 (ASG) radar, 39 
AND (admiralty net defense), antitor- 
pedo device, 30, 41, 98-99 


Antisubmarine equipment 
antitorpedo nets, 30, 41, 98-99 
Asdic ecpiipment, 8, 16, 60 
bearing deviation indicator, 59-60 
British Squid, 60 

depth charges, 9, 12-13, 59-60, 74-75 
expendable radio sono-buoy, 41 
Hedgehog, 21, 30, 52 
high frequency direction finding, 13, 
17,27,41 

Leigh searchlight, 30, 40 
magnetic airborne detector, 30-31 
* “mousetrap” projector, 30-31 
PPI scopes, 29 

Q attachment (depth-angle sonar) , 41 
radar, 14, 30, 40-41 
towed noisemakers, 162-171 
Antisubmarine measures, 89-91 
convoy escorts, 90-91 
evasive routing of convoys, 90 
high ship speed, 95-97 
objective of antisubmarine warfare, 89 
safety of independent shipping, 93-99 
submarine operations, 89-90 
Antisubmarine warfare, effect of end-of- 
war and future developments, 
178-179 

Antisubmarine warfare, history, prewar 
to September 1939; 1-2 
convoys, 1 

lack of satisfactory countermeasures, 1 
need for scientific and technical ad- 
vice, 2 

submarines in World War I, 1 
Antisubmarine warfare, history, Septem- 
ber 1939-June 1940; 3-7 
aircraft, 5-6 
convoys, 4-5 
sinking of U-boats, 6 
surface craft, 6 

Antisubmarine warfare, history, July 

1940- March 1941; 8-15 
aircraft, 12-13 

convoys, 11-12 
sinking of U-boats, 14 
surface craft, 14 

Antisubmarine warfare, history, April 

1941- December 1941; 16-24 
aircraft, 20-21 

convoys, 20 

sinking of U-boats, 22-23 
surface craft, 17 

Antisubmarine warfare, history, January 

1942- September 1942; 25-33 


aircraft, 29-30 
convoys, 28-29 
sinking of U-boats, 30-31 
surface craft, 22 

Antisubmarine warfare, history, October 

1942- June 1943; 34-43 
aircraft, 39-40 

convoys, 38-39 
sinking of U-boats, 41 
surface craft, 40 

Antisubmarine warfare, history, July 

1943- May 1944; 44-63 
aircraft, 54-56 

convoys, 53-54 
sinking of U-boats, 60 
Antisubmarine warfare, history, June 

1944- August 1945; 64-79 
aircraft, 73-74 

convoys, 72-73 
sinkings of U-boats, 77-78 
Antisubmarine warfare, summary, 80-87 
Antitorpedo nets, 30-31, 41, 98-99 
Asdic, British sonar equipment 
depth predictor, 50, 60 
in minesweepers, 16 
limitations, 8 

ASV (air-surface vessel) radar, 14 
Attack against submarines, theoretical 
analysis 

attack errors, 114-119 
by aircraft, 127-138 
by surface craft, 110-126 
factors determining success, 114-115 
search and interception, 139-152 
weapon lethality, 119-120 
Attack errors 
aiming errors, 129-130 
aircraft attacks, 128-130 
blind time, 114-115, 128 
estimation of submarine position, 
128-129 

factors influencing errors, 122-124 
sonar errors, 114-119 
surface craft attacks, 114-119 
variation in behavior of weapons, 128- 
129 

BDI (bearing. deviation indicator), 59-60 
Blind bombing aid, 57 
Blind time, antisubmarine attacks, 114- 
115,128 

Bombing errors, aircraft attacks, 137-138 
Bombs, jet-propelled, German, 45 
Borkum, German search receiver, 57, 156 


191 


192 


INDEX 


British iintisubniaiinc c(|iii|)nicnl 
Asdic, 8, !(),()() 
depth charges, 137 
FOXER, 48, 55, 163 
radar, 14, 21-22 
Scjiiid, 60 

strapped magnetron, 40-41 

C^amonflage on ll-hoats, 159-160 
C;AP (Civilian Air Patrol), 29 
Convoy defense, 100-112 

against Schnorchel-equipped U-boats, 
72-73 

air-escorted convoys, 39, 109 
ellect of convoy size, 106-107, 109-111 
elfect of convoy speed, 104-106 
end-to-end escort of convoys, 20 
gain in safety, 100-104 
limitations, 111-112 
radar equipped, 1 1 
surface escorts, 107-109 
Countermeasures to the acoustic tor- 
pedo, 161-176 
anti torpedo nets, 98-99 
towed noisemakers, 162-176 
types, 161-162 

Countermeasures to the U-boat 
see Antisubmarine equipment 

Damage from U-boats 
see Antisubmarine warfare, history 
Depth charges 
air use, 12-13 

comparison of British and U. S., 137 
depth setting, 21, 136-137 
Mark 8; 59-60 
Mark 9; 60 
Mark 14; 74-75 
torpex- filled, 30 
Depth predictor. Asdic, 50, 60 
Direction finding, high frequency, 13, 
17, 41 

DMS-1000, microwave radar, 39 

Electric driven U-boats, 177 

ERSB (expendable radio sono-buoy), 41 

Fast time constant (FTC) circuit, 74 
FAT, German torpedo gear, 56 
Foxer, British noisemaker, 48, 55, 163 
FXR, U. S. noisemaker, 48, 56 
effectiveness, 163, 172-173 

German 

acoustic torpedo, 48, 161-176 
electric driven U-boats, 177 
Geier, homing torpedo, 75 
Gnat, acoustic torpedo, 173-176 
Hohentwiel, aircraft warning radar, 
153 

human torpedoes, 52 


jet-j)ropelled glider bond), 45 
Kurier, high speed communication, 75 
Uiri', torpedo gear, 75 
Pillenwerfer, submarine bubble tar- 
get, 30 

radar countermeasures, 151-160 
search receivers, 30, 55, 154-158 
turbine driven U-boats, 177 
U-boat tactics, 177 

German U-boat losses in World ^Var II, 
181 

Glider bomb, jet-propelled, 45 
Gnat, German acoustic torpedo, 173-176 
description, 172-174 
effectiveness, 176 
speed, 56 

Guided missiles, effect on antisubmarine 
warfare, 178 

HoS, blind bombing aid, 57 
ffedgehog, multi-spigot mortar 
design, 21 
effectiveness, 52 
torpex-filled, 30 

HF/DF (high frequency direction find- 
ing), 13, 17, 27, 41 

Hohentwiel, German aircraft warning 
radar, 153 

Homing torpedo, German 
see Acoustic torpedo, German 
Human torpedo, German, 52 

Italian submarine losses in World War 
II, 181 

Japanese submarine losses in World War 
II, 181 

Jet-propelled glider bomb, 45 

Kurier, German high speed communica- 
tion, 75 

I.eigh searchlight, 30, 40 
LUT, German torpedo gear, 75 

MAD (magnetic airborne detector), 31, 
50-51 

Magnetron, strapped, 40-41 
Mark II radar, 21,30, 39 
Mark HI radar, 40 
Mark 8 depth charge, 59-60 
Mark 9 depth charge, 60 
Mark 14 depth charge, 74-75 
Metox, German search receiver, 40-41, 
154-155 

“Mousetrap” projector, 30 

Naxos, (ierman search receiver, 55, 57, 
157-158 

NM (towed noisemaker), 162-171 
description, 162 


disadvantages, 161-162 

effect on trajectory of torpedo, 169-171 

FOXER, 48, 55, 163 

FXR, 48, 56, 163, 172-173 

Operation Torch, 34-35 

Pillenwerfer (submarine bubble target), 
30 

Plan position indicator, 29 

Projector, sonar, 4 1 

Q attachment, sonar depth-angle pro- 
jector, 41 

Radar, enemy countermeasures to, 151- 
160 

aircraft warning radar, 153 
camouflage, 159-160 
decoys for radar, 154 
Schnorchel, 158-160 
search receivers, 30, 55, 154-158 
submergence of submarine, 153 

Radar detection of U-boats 
airborne radar, 13, 151 
detection of Schnorchel, 159, 176 
effectiveness, 151-157 
enemy countermeasures, 57, 151-160 
fast time constant circuit, 74 
Mark II radar, 21, 30, 39 
Mark HI radar, 40 
SCR-517 radar, 39 
shipborne radar, 27 
type 271; 21-22 
type 286M, 14 
Vixen attenuator, 157 

Radio sono-buoy, 41 

Radio-controlled jet-propelled glider 
bomb, 45 

RD/F 286 M radar (radio direction-find- 
ing), 14 

Rocket projectiles, 56 
effectiveness in aircraft attacks, 138 

SADU (seasearch-attack development 
unit), 29 

SBT (submarine bubble target) , 30 

Scarecrows, aircraft used against U-boats, 
6 

Schnorchel on U-boats, 58-59, 64-78, 
158-160, 177 

countermeasures to, 159, 176 

SCR-517 radar, 39 

Search receivers, German, 30, 154-158 
Borkum, 57, 156 
Metox, 40-41, 154-155 
Naxos receiver, 55, 57, 157-158 
Tunis receiver, 75-76, 158 
WanzG-1,57, 156 

Seasearch-attack development unit, 29 


Shippijig safety nieasmcs, 

loclutlion in submarine’s ability to ap- 
proach ships, 95-98 
reduction in snl)marine's ability to 
contact ships, 93-94 
Sonar gear, limitations, 114-116 
Sonar projector, depth-angle, 41 
Sono-buoy, dropped from aircraft, 41 
Squid, projection of depth charges, 60 
Submarine, countermeasures to 
see Antisubmarine equipment 
Submarine attacks, enemy 

see Antisubmarine warfare, history 
Submarine bubble target, 30 
Submarine search and interception, 139- 
152 

aircraft search for surfaced submar- 
ines, 139-141 

estimation of submarine position, 128- 
129 

follow-up of contacts, 147-152 
interception of transits, 143-147 
radar versus visual search, 140 
surface craft search, 141-143 
Surface craft attack on submarines, 110- 
126 

attack errors, 114-119, 122-124 
barrage lethality, 120-121 
calculation of probability of success, 
121-122 


INDEX 


coordinatetl \ersus independent at- 
tacks, 125 

effect of sonar conditions, 123 
effectiveness of attacks, 122-126 
factors determining success, 1 14—122 
listening and echo-ranging attacks, 
123 

weapon lethality, 119-120, 122-126 
Surface craft search for submerged sid)- 
marines, 141-143, 148-151 

T-4 acoustic torpedo, 171-172 
T-5 acoustic torpedo, 173-176 
description, 172-174 
effectiveness, 176 
speed, 56 

Torpedo, acoustic, German 
see Acoustic torpedo, German 
korpedo trajectory analysis, 164-171 
effect of noisemaker, 169-171 
flip-flop rudder control, 167 
sensitivity pattern template, 166 
Torpedoes, human, 52 
Torpex-filled projectiles, 30 
rowed noisemakers, 162-171 
description, 162 
disadvantages, 161-162 
effect on trajectory of torpedo, 169-171 
Foxer, 48, 55, 163 
FXR, 48, 56, 163, 172-173 


193 


Tunis, (German search receiver, 75-7(), 
158 

rurl)ine driven U-boats, 177 

U-boat countermeasures 
see Antisulmiarine ec[uipment 
U-boat losses in W^orld \V^ar II 
■ German, 181 
Italian, 181 
Japanese, 181 
U-boat warfare 

see Antisubmarine warfare, history 
U-boat weapons 

acoustic torpedo, 56, 98-99, 161-176 

aircraft warning radar, 153 

FAT, torpedo gear, 56 

Kurier, high speed communication, 75 

LUT, torpedo gear, 75 

Pillenwerfer, 30 

radar decoys, 154 

Schnorchel, 58-59, 64-78, 158-160 
search receivers, 30, 55, 57, 154-158 

\'ixen attenuator, radar, 157 

\’LR aircraft (very long range), 35-36 

^\’alter turbine propulsion for submar- 
ine, 177 

^Vanz G-1, German search receiver, 57, 
156 

Weapon lethality, 119-120, 130-132 






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